LaRue,
This is very simple. Lets start off making a simple problem...
Lets say we have a simple fractional number...
3/32=.09375
To convert back to a fraction, just multiply .09375 by the denominator you are looking for. So .09375*32, you end up with 3.
Since we got that figured out, lets look at your number.
0.6496063 inches. Looking at the number, we know it is larger than 1/2" and smaller than 3/4".
.6496063 is a fraction. Start with a larger denominator like 64 to get as accurate as we can be.
0.6496063*64=41.5748/64"
41.5748/64=20.787/32=10.39/16=5.19/8
So in the end, a 5/8" hole would be slightly smaller than you are looking for. So, 21/32" is the closest size in 32nds and 11/16" is the closest in 16ths.
Pretty simple...huh?
Tom