I suppose I should say I made an elaborate jig and all that, but honestly I just taped a bottle cage on the frame, marked the locations and carefully drilled the pilot holes.

I measured a few frames with factory brazed bosses, and there didn't appear to be a standard location for bottle mounts on the downtube. You do need to be careful, though, to put the bottom hole high enough that you can get your drill in between the seat tube and downtube. My bottom hole is about 6" from the bottom bracket. A right-angle drill would be helpful but I don't own one. If you plan to do a lot of these you could cut off a drill bit and re-grind the edge; that would buy you some flexibility.