They are usually obtained from your board house (common sizes are free, odd values are available but usually with an extra tooling charge for mounting the non standard drills. I use this one from AP Circuits
Free sizes:
.0197
.0280
.0350 ic pins
.0380 .25 connector posts
.0420 1n4001 diode holes
.0520
.0595
.0860
.1250
.1520
the ones with text are what the Fritzing parts factory amd/or core parts use for the listed part. As far as I know they aren’t documented anywhere except the source code. Your best bet for drill sizes is to check with who is going to make your boards, or if you are doing it yourself what drill sizes you have. Your calculation looks right but the best way to check is to export the gerber files then edit the drill.txt file which will tell you what drill sizes are being called for, you can check the pad size in a gerber viewer such as gerbv (it has a measurement function).
Peter