I kmow it's probably somewhere, but can't find it, so... starting a thread for nothing but builder's tips, tricks, hacks, fixes and shortcuts.
Here's one. When locating tuners on a tight fitting headstock, cut small pieces of painters tape the size of the tuner's base plate and organize on the underside of the headstock. Once you're happy with layout, use a scribe,icepick or small nail to poke a hole in the middle of each tape. You now have exact position for drilling post holes