I tend to look at the entrance of the canal. I check to see if I can see the dot! It I can, and I am on the last dot, I push a little harder. So far, still maintaining depth with all dots in. I would imagine that labia thickness varies so we would need to consider the canal approach. As for me, I still seem to be a bit puffy so mine are thicker. I suspect that this may be the case with cis women as well.
All the Best! Hugs
Kelly