just put sonic springs in my forks a couple weeks ago. I bought the for level guage but could have just as easily done it with a piece of wood etc 110mm long and a turkey baster. sonic includes really good instructions maybe they have them on there web site. measure with the springs out fork compressed from the top of the tube make sure you cycle the forks a few times when adding the oil to work the oil through the forks and bleed the air out
Stock level is 100mm below the top of the fork tube, spring out, forks compressed. Many folks run 110mm. Up to you.
Take a rod and slide it along the front of the fork tube, extract and measure dry length. Do the same on the back side of the fork tube and average the two. It doesn't have to be exact. +/- 1/4" is plenty close enough.