Measuring from the tail works, but the measurement will be different for each length of a ski. You have to have the number for that length ski.
Another way is distance from the center of the ski. Blister Review uses this number. I am not sure, but that might be the same number for all lengths. It might not be either.
IMHO, there would have to be different recommendations for mount point by whatever method, for different lengths (from factory line, ski center, distance to tail, etc.). In my experience, different lengths of the same model ski
usually have different optimal mount points
for the same skier relative to factory line, ski center, or whatever other reference point one uses. This is mostly because the same skier will find differing fore-aft dynamics, dampness, etc. versus quickness or other trade-offs, on the same model at different lengths.
For example, with a 189 Shreditor ski, I may find fore-aft is so stable with the extra length that I can comfortably move forward of factory line to gain easier turning/playfulness; while with a shorter Shreditor, say 179, fore-aft stability considerations may override any gain in turnability/playfulness, so that I would tend to stick with factory line or even go farther back.
There are other trade-offs with mount point also that would change optimal mounting point(s) for differing lengths - partly because the height/weight/style of each skier stays constant, while the length, weight, radius, shape, etc. of the ski changes.
