Mowing calculator
One rule does most of the work in mowing: never take off more than a third of the blade in a single cut. This tells you when that means mowing, and what a cut you are planning would actually remove.
What the rule says
That is more than a third. Cut to 2.67" first, let it recover for a few days, then take it lower.
Why a third
The blade is where the plant makes its food. Cutting deeply removes so much of it at once that the plant stops putting energy into roots and spends it re-growing leaf — which is why scalped lawns brown off, thin out, and then lose the bare ground to weeds. Taking a third or less keeps the plant working on its roots.
Coming down from a long lawn is therefore a series of cuts, not one. Each pass may take a third of what is standing, so let it recover for a few days between them.