When I want to get a chironomid to a certain depth, I clamp my forceps to the hook, lower the line to the bottom and reel in so the hook will be above bottom by a foot or two then I use a whiteout marker on the line at the first guide. The stuff will wipe off without much trouble and is persistent when wetted. Not sure if that will help you or not.I am not as interested in measuring casting distance as I am in trying to reach a particular depth when using sinking lines. When I use a depth sounder or anchor rope that tells me the bottom is 30m down and I am trying to get down there to that depth, I want to be there. I often cast as far as I can, then let line pay out even until I reach backing, do some mental guestimation based on what I think is the amount of line out and an estimate of the IPS sink rate of the line and the angle the line, having a good estimate of the amount of line out would aide this.