The one rule that prevents most errors

Never type a number inside a formula. Every assumption — the fee, the rate, the number of months, the exchange rate — gets its own labelled cell, and every formula points at those cells. This one habit prevents the majority of spreadsheet errors made by everybody, at every level of experience.

The reason is that hidden numbers cannot be found or changed. A fee buried inside four formulas will be updated in three of them next year, and the sheet will produce a wrong answer confidently and silently. Put the assumptions in one block at the top, label them in words, and the sheet becomes something you can still trust in six months.