Formal and executable

States the rule as inputs, an operation, and an output, so the same transformation runs the same way twice.

You have explained your rule for what a freelance day is worth to three different people this year. Each time it came out a little differently, and each time you landed on a different number. Nothing is wrong with the thinking. The thinking just lives in your head, so you re-improvise it every time somebody asks.

Formal and executable writes the rule down in a form that runs the same way twice. It always has the same three parts. What goes in, what the rule does to it, and what comes out. Once those three are separate and written, the rule stops being something you remember and becomes something you can hand over. Another person can put their own numbers in. They can point at one step and say that step is wrong, which is a thing nobody can do to a hunch.

It has six renderers. An equation states the relationship in symbols, so values can be substituted under each one. Code is the version a machine runs, which forces every case to be handled. An algorithm is numbered steps with a stopping condition, runnable in your head with no machine at all. Notation is a compact symbol set plus the legend that decodes it. A spreadsheet puts the rule in a cell, so changing one input moves everything downstream. A ledger records dated entries with a running total that has to reconcile.

Use it when the same calculation keeps coming up and keeps coming out differently. It is weak on tacit meaning and weak variables. If a quantity cannot be defined, an equation over it will look exact and mean nothing. The specific danger is that this form reads as certain. The rule runs whether or not the assumption sitting inside it is true, and it runs with the same confidence either way. So write the assumptions beside the rule, in the same eyeline, or the form will hide the one thing you most needed to see.