Ask for the formula, never for the total

Why it is good at formulas and bad at sums

These look like the same task and they are opposites.

A formula is text with a rigid structure: a pattern the model has seen hundreds of thousands of times. Producing =SUMIFS(D:D, B:B, "Ikeja", C:C, ">="&DATE(2026,1,1)) is language work of exactly the kind it does well.

Adding a column of 300 numbers is arithmetic. Nothing in generating likely text performs a carry. What comes out is a number of about the right magnitude, produced by the same process as everything else, with no signal that it is wrong.

So the division is clean, and it is the most useful single habit in the back office:

The model writes the formula. The spreadsheet does the sum.

What this unlocks

Most owners use about six spreadsheet functions and avoid the rest, which means real analysis never happens. That barrier is gone.

Describe what you want in words.

Column A: date. Column B: customer. Column C: area.
Column D: amount. Column E: paid yes/no.

Give me a formula for total unpaid, by area, for the last
90 days. LibreOffice Calc. Explain what each part does.

Ask it to explain a formula you inherited. Paste the incomprehensible one from the sheet your predecessor built. This is a good use: it will explain it, and it will often point out that the range is wrong.

Ask for the right function. “I want to look up a price by product code” gets you XLOOKUP with an explanation of why it is better than VLOOKUP, which is the sort of thing nobody has time to look up.

Get a pivot table set up. Describe the summary you want; get the steps.

The three-row test

Formulas fail silently. A wrong range, an off-by-one, a lookup returning approximate matches: the sheet fills with numbers and nothing looks broken.

So before trusting any new formula: build three rows where you know the answer, by hand. A row that should return 100, a row that should return 0, and an awkward one: a blank, a zero, a date at the boundary of the period.

If all three are right, the formula is probably right. If the awkward one is wrong, you have found the bug in ten seconds, and it is almost always at a boundary.

This takes two minutes and it is the entire safety procedure. Skipping it is how a business reports the wrong figure for four months.

Free tools

LibreOffice Calc: free, open source, offline, opens and saves Excel files. Functions and formula syntax are close enough to Excel that advice for one works for the other.

Google Sheets: free, shared, works on a phone, and has AI features built in that keep your data inside a service you already use.

Excel if you have it. Job ads name it, so it is worth being able to say you use it, and everything here transfers.

Note that the AI features inside the spreadsheet are often better for this than a chat window, because they can see the sheet (the column headings, the data types, the actual ranges) rather than your description of it.

The boundary, restated

Formulas: yes. Explanations: yes. Structure and layout: yes. Finding the likely error in a sheet that does not balance: yes, and surprisingly good.

The value of a cell: no. A total in a chat window: no. A figure quoted in a report because the model said so: no.

And the reconciliation rule from the bookkeeping lesson still stands over all of it. Whatever the spreadsheet says, the bottom line matches your bank statement, or the spreadsheet is wrong until proven otherwise.

Describe the sheet, not just the task

The commonest reason a formula comes back wrong is that the model was guessing at your layout.

Give it the shape first: which row the headers are on, what each column contains, what the data types are, whether there are blank rows, whether the dates are real dates or text. Paste five representative rows, with any awkward ones included.

Row 1 is headers. Data starts row 2, about 1,400 rows.
A: date (real dates, dd/mm/yyyy)
B: customer name (text, some blanks)
C: area (text, one of six values)
D: amount (number, some negative for credit notes)
Sample rows:
02/03/2026, Adeyemi, Ikeja, 14500
05/03/2026, (blank), Yaba, -2300

Those negative credit notes are exactly the sort of thing a formula written without them will get wrong, and exactly the sort of thing you would not have thought to mention.

Getting help with a sheet that will not balance

A specific and very good use. Describe the symptom (the total is out by 4,380 and you cannot see why) and ask for a list of the likely causes in order.

You will get the usual suspects: a range that stops one row short, text stored as numbers, a hidden row, duplicated entries, a sign error on refunds. Then check them yourself. It is a diagnostic checklist rather than an answer, and a checklist is what you actually need at eleven at night.