I count from when the weight is in the water while I'm going at the prescribed speed. I calculate that if my line is close to a 45deg angle from the rod tip to surface, then I multiply the line length below the water by 0.6 and that gives me a ballpark depth. SO, if I let out 20ft below the surface, then I estimate my weight is about 12ft deep (& my crank would be a foot or two below that, depending on lip size of crank and length of leader). (20x0.6=12 .... or you can use the "every 5ft of line out equals 3ft of depth" calculation) If I'm not mistaken, I believe the trolling charts are based on the same calculation principle. (2oz weight @ 1.8mph w/45deg angle of line ... though I'm not sure if it's with a 3ft leader or 5ft leader)