One "relatively" safe means to measure the insertion depth of a late model Visual Paracorr while at its lowest height setting on the tunable top would be to simply place a cotton 'Q'-tip down the side until it rests by gravity on the lens element, and then mark it at the shoulder junction. Then remove it, and stand it up (without any downward force applied), and measure from the table top to the mark on the 'Q'-tip.
CAUTION / WARNING: Again, I warn that I would not wish or intend that anyone scratch a Paracorr in attempting to measure it.
-------------------- My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.