If it doesn't work on the back of an envelope, it won't work in the spreadsheet.
A spreadsheet is a wonderful machine for making a bad idea look survivable. You add a tab. You add an assumption. You colour a few cells green. By row forty you've built something that projects its way to profitability through sheer force of formatting, and somewhere along the line you stopped noticing that the core maths never actually worked.
The back of an envelope can't do that. You've got a corner of paper and a pen. Price times customers, minus what it costs to serve them. If the answer is grim, it's grim in about thirty seconds, and no amount of conditional formatting will save it.
I do the rough version first now. Always. Before the model, before the deck, before I've fallen in love with the idea. Crude numbers, big round figures, the kind of estimate that's wrong in the details but right about whether the thing can fly at all.
If the rough maths says no, the detailed maths is just a more expensive way to arrive at the same no.
The envelope tells the truth faster than the spreadsheet.
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