I used a permanent marker. It's in the 'blind' spot along with the reinforcing ring.
The teflon pads are a must. Do not replace with anything but teflon.
The transparency will not damage the mirror, but if poss., get someone to hold it for the second it takes you to 'make your mark'!
ps. If people are having trouble with the balance of their tubes, experiment with the position of the alt teflon pads. Move them up in the 'D' to tighten, & down the 'D' to free up the movement. (about 1/4" to 1/2" is all you need). You might just be able to get rid of your dreaded 'Springs'!
HTH.
L.