That is almost the exact way i located the screw holes, but my case had a removable mobo tray that made it a a bit easier.I'm just planning to stick my case on it's side, lay the motherboard down, and mark the missing holes.
If you look at your motherboard tray you will usually see inserts for the threads used for stand-offs, Without the inserts most of the time you have just a few threads on the tray, which makes it really easy to strip.
0dd