Homebrew PCBs

I tend to use resharpend drills (as they are cheaper and so am I !)
but 
use a collar to set the length to a standard so changing mid way 
through a board is less hassel.

The 1/8" drill shank has a 3mm collar forced over it so is a very
tight 
fit. 

Actually adjusting the collar to sit so that the drill tip (or more 
critically a fishtail cutter tip)is exactly where it should be is a
bit 
hit or miss and I wondered if anyone else uses this method and if so 
has a magic way to adjust them.

I sat down to design a screw device to force the drill through the 
collar until it touches a contact and lights an led, but then
realised 
that the collars vary in thickness and the critical reference is from 
the TOP of the collar to drill tip.

AWEM