I've always gone low tech. Mark # of pints until you get ~ 40, so I don't have to lift and stir up sediment. Bigger glasses tend to throw it off quite a bit though!
+1. I have a blank spreadsheet with lines for about 10 kegs, I tick off in columns for growlers, pints, 12oz bottle fills, etc. I usually forget to mark a growler in there somewhere... but it gets me close.
I used to use a bathroom scale calibrated at zero for an empty keg, but once I expanded above 2 serving kegs (I often have 5 or 6 on tap now) I dropped that idea.