http://www.gruhn.com/features/55tele/EF7345h.jpg
I usually just eyeball it, and then loosen the 1st and 2nd strings a little, hook the strings under the retainer, and push it down until it's resting on the wood. Once it looks like it's in the right place, take an icepick (or something equally pointy), put it through the screwhole in the retainer, and make a mark for your drill bit.
Make sure you measure how deep the screw will go, and mark your bit with a piece of tape as a depth-marker. I always mark my bit a tad less than the length of the screw. Make sure you measure WITH the screw in the retainer. It takes VERY little pressure to go straight through the headstock with an electric drill, which is why I have an antique hand drill. When you're drilling holes that are only 1/8 to 1/4 inch deep, you really don't need electricity.