[greyhair]

> sanity-check intermediary calculations

I am very far from being an Excel guru, very far, but one thing I have found useful is having everything come out to an intermediate result, and calculate against those intermediate results. If you see weird numbers in the middle, then end result is likely wrong, figure out why your intermediate results are messed up. If you want to be fancy, you can even error bound your intermediates if you know your valid ranges.