How do you measure for a bit?

Cocorules

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When you do not already have a bit to use to check for size, how do you measure?

If for example from outside edge of lip to outside edge of lip (worked out from holding a tape across the front of the nose) it is 5" do you add 1/4" or 1/2" on top? Or is there a better way to do it?
 
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