Contingency self-check
A flat 10% is the internet's answer. Your real number depends on your site, your documentation and your experience — this check gives you the buffer your risk profile actually needs.
How to read your result
If you're showing a shortfall, don't shave the number to make the spreadsheet feel better — the risk doesn't read spreadsheets. Either fund the gap before you start, or reduce the risk that drives it: finish documentation, get the soil report, lock selections. A contingency that only exists on optimistic days is how 10–20% blowouts start.