Check the mirror for boring true and square
Most of us believe that mirrors don't tell the truth on their best days. But when I need to drill a perpendicular hole and can't use a drill press (such as in the middle of a large workpiece or on a wall), I use a piece of mirrored acrylic with common hole sizes predrilled through it. When the image in the mirror and the drill bit align, the hole is perpendicular. Just be sure to view the bit from both front-to-back and side-to-side orientation.
I keep my drill mirror in my toolbox. To avoid scratches during storage, I made a cardboard sleeve.
—William Shockey, Portland, Ore.