Are you trying to measure the spindle bearing face width to see if it's Italian or English or trying to determine the length for correct chain line?
Are you wanting to measure the bb cup diameter or the thread pitch?
Is you BB shell 68mm wide (English) or 70mm wide (Italian)?
The Italian BBs also has a slighty bigger inside diameter and are right hand threaded, not left hand threaded (reverse) as English and french BBs are, (the vast majority).