Free tool
Estimation Bias Calculator
Log a few tasks where you guessed how long something would take and then timed how long it actually took. This tool averages the gap and gives you one number — a personal correction multiplier — to apply to your next gut estimate.
Per task
You just did by hand what Groundkeep's task tracker does automatically for every task you complete. Give a task an estimateMin, time it with the built-in focus timer, and Groundkeep sums the real work time and compares it against your estimate on its own — no separate spreadsheet, no re-typing numbers into a calculator after the fact. Over enough tasks that becomes a running estimation history you can actually look back on, broken down per project or per kind of work, instead of one gut-feel average from three examples.