On the 327 I used, you measure it from the flange at the bottom of the taper to the end of the bit, and put that in the tool table.
Then, we'd have a program to route 5mm deep in a 15mm board, just a short route. Measure the remainder (it should be 10mm) and adjust by the difference. If it cuts too deep, use a larger number. If it's not deep enough, use a smaller number.