Duane Bozarth wrote:
G Henslee wrote:

A trick is to use a mirror and a square...

Does it need to be a square mirror?


Actually, the easiest way is to use a folding mirror at 90-degree. Set
a small square where is reflected in both and line the drill up against
its relection in the mirror by sight--can get quite accurate in both
directions.

Hmm, I figured, round hole - round mirror.