Originally Posted by
cycle_maven
I've always found it more accurate to measure the circumference and divide by pi, then subtract twice the depth to the nipple slot. But this requires an accurate flexible tape measure, which are sometimes a little hard to come by...
Sutherand is offering a tape measure depth gauge set exactly as you describe. I believe the tape calibrated in diameters so you don't even have to divide by Pi. (if it isn't he missed a good move) All this for the bargain price of USD $75. I don't own anything dedicated to this job, so use whatever steel tape is handy, and make a depth gauge from a piece of cardboard, which I mark and measure.
Been doing it this way for almost half a century and it still works fine, though when everything is computerized, and pens no longer made, I might have to find a new depth gauge.