Only Use a Digital Caliper To Check What Size You May Own

Example of how to measure on the left.

You're going to measure from all 3 points to check the size, there may be a small amount of variance in different digital calipers, Just go with the number that closest to your bit, Meaning if you measure 4.88mm or 4.92mm, Its Most likely a 4.9mm (Providing You Measured Correctly from 0)

Only Use a Digital Caliper For This.

Attempting to correctly Measure a screw for a bit to purchase is much more difficult.

The Way I Do it; First i preset my caliper to about the size of a bit i believe the screw to Be, In This Example we are guessing a 3.5mm 

Then i Hover The Caliper Over The Knifes screw and check, If i was correct, it would look like im measuring JUST The Holes... 

As We Can See In This Example a 3.5mm Would Be Too Large for this knife...

Well We know a 3.5mm is too big, so lets move down to 2.5mm and check again

As we can see in this example, The correct choice was 2.5mm (Flat, cause its flat)

Yep its a 2.5mm Flat