You should not measure the bit shaft, You should cut a slot in some kind of hard stock (maple, ect.) and measure the slot. Bits are made by starting with a rod and sharping the flutes into them, so the odds of them being the same size as the shaft are not good.
I have had good luck with telling the bot that my bits are .245"
Kyle Stapleton
River Falls Renaissance Academy
Math/Technology Education Teacher
PRS Alpha 96x60 2.2 hp spindle, Double Air drills, 6" indexer, Fein 5 zone vac table
Desktop w/spindle
Potter Pen
Aspire 8.5, Creo 3.0