But I use a piece of clear plastic tubing marked with the air gap, and my trusty mini-vac vacuum pump to draw off the excess. There are tools you can buy to do this with an adjustable ring on a thick needle (SS tube) that you can set to the air gap and then siphon off the excess to get the desired air gap. Regardless of whether you are renewing the oil or filling from 'dry', the key is to add (or draw out) enough oil to leave a specific 'air gap' above the oil level in each side to +/- 1/8'. If 'dry', then add the amount shown in the book and then do as below.
If you are just changing the oil, the best idea is to measure how much oil you drain from each side, and start with that much on the make-up. The exact amount of fork oil you need will depend on whether you are just changing the oil or you have disassembled the forks and are filling them from 'dry'.