From $110 · fixed price

Custom calculation model

A spreadsheet built from your source document, with every figure traced back to the page it came from.

See the three tiers Or email first

What this looks like

The same job, in four pictures

Headed “Close, not correct”: a spreadsheet grid where one cell, =B7*0.0375, is marked in coral and labelled “rate typed, not read”. The line below says the build is checked against your source document. Headed “In every build”: six coloured cards listing what a build includes — checked to your source cell by cell, inputs in one place with formulas locked, a check tab that proves the totals, no hidden numbers inside formulas, written notes on every assumption, and zero, blank and negative all tested. Delivered in 3 to 10 days. Labelled “Illustration — not a client result”: two cards side by side. Twenty-three lines off against the source on the first pass, and none left unexplained at delivery. The line below says the numbers shown are not yours. Headed “Tested against a published table”: the figure 4,350 in a large violet panel. It is the number of published cells in a public IRS table that a payroll withholding calculator built here was checked against, across 13,815 formulas. Your calculator gets the same check, against your own document.

The before-and-after figures in these pictures are illustrations, not a client result. Your numbers and your source are the only ones that end up in your file.

WHAT IT IS

A model with a paper trail

You have a document that holds the rules: a rate card, a published table, a contract schedule, a regulation. Someone has to apply it repeatedly, at speed, without re-reading it each time. This builds the workbook that does what the document says, plus the log showing where each figure came from.

The build is not the hard part. Plenty of people can produce a file that returns numbers. The difference is that when somebody later asks why a particular figure is what it is, there is a line in the log naming the document, the page and the edition date, with the arithmetic between the source and the cell written out.

THE SOURCE

What counts as a usable source

The model is only as defined as the document behind it. A usable source is one where two people would extract the same figures.

  • Published tables and schedules: rates, bands, tariffs, fee scales.
  • Contracts with defined calculation clauses.
  • Regulations and official publications with the figures stated in them.
  • Internal rate cards and price lists, provided one version is authoritative.

TRACEABILITY

Every figure has an address

What does not work is a source that lives mainly in someone's head, or four spreadsheets that disagree. Where that is the situation, the first job is settling which document wins. That decision is yours; I will show you where the versions conflict.

Traceability is built during the work rather than written up afterwards. Each figure the model produces gets a row in the reconciliation log as it is built.

  • The output: sheet, cell, and what the figure represents.
  • The value the model produces.
  • The value stated in the source.
  • The source's title, page or table reference, and edition date.
  • Match, or mismatch with the difference.
  • For derived figures, the arithmetic connecting source to output, written out.

Where the source states a figure, the model reproduces it exactly. Where the model derives one, the derivation is shown so it can be argued with.

WHAT YOU RECEIVE

The file and the evidence

Delivery is the workbook and the material that makes it checkable.

  • The workbook, with inputs separated from calculation and calculation separated from presentation.
  • The reconciliation log, inside the file, with one row per figure.
  • A note of the exact source edition the model was built against, so you know what to re-check when a new edition appears.
  • A list of anything the source does not settle, with the assumption used instead, stated as an assumption.

AI

AI builds fast and invents quietly

AI tooling is used in the build, and I would rather state that than be asked. It is quick at structure and quick at formulas.

It will also produce a bracket that looks like the published one, a rate near the real one, a rounding rule that fits the pattern of the surrounding rows and appears nowhere in the source. That is the reason the reconciliation exists in this form. Every figure in the finished model is checked back to the document and recorded with page and date, so what you are buying is not confidence in the builder. It is a file you can verify without me.

AI disclosure

I use AI tooling in the build. That is exactly why every figure is reconciled against your source and why you get the log.

BEFORE YOU SEND ANYTHING

Your file stays yours

A workbook you send is used for the job you asked for and for nothing else. It is never shared, never used as an example, and never kept as a sample. My copies are deleted as soon as the work is delivered.

If you need a non-disclosure agreement signed before you send anything, say so and it gets signed. It costs nothing and it is a normal way to start.

One thing I do not control, and would rather say than hide: the file travels through Upwork, so Upwork keeps its own copy under its own retention policy. What I can promise is my copy, and that one goes.

Priced and ready to go

Three tiers, fixed price

Starter

$110

3 days

Standard

$395

5 days

Advanced

$1,020

10 days

No calls needed. Describe what you need in writing and you get a price and a date the same day.

Open it on Upwork

Questions

Asked before hiring

What source documents can a calculation model be built from?

Published tables and schedules, contracts with defined calculation clauses, regulations and official publications, and internal rate cards where one version is authoritative. The requirement is that two people reading the document would extract the same figures from it.

How do I check the model is right?

Open the reconciliation log inside the workbook. Each figure has a row giving the value the model produces, the value the source states, and the source's title, page reference and edition date. Pick any row and repeat the check against the document yourself.

What happens when the source document is updated?

The delivered file records the exact edition it was built against. When a new edition appears, the reconciliation can be re-run against it, and the log shows which figures moved and which did not.

Is a custom calculation model financial or tax advice?

No. It is an engineering service. The model reproduces the document you supply. Whether that document is the right one to apply, and what the outputs mean for your business, are questions for a qualified adviser.

A model built this way, in public

The payroll engine is one of these, built to the limit: 9 tabs, 13,815 formulas, 4,350 figures reproduced from IRS Publication 15-T to the dollar, with a 4,375-line substantiation log naming the source, page and date of every figure. It is the specimen. Open it and check a row.

Bring the document. The model is built from it, not around it.

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