I suppose the bulls eye level should work, as long as the timber is true and level.

I use a combo square set to 0, registered in two directions and let it rip. this works as long as the timber is not out of square. I also bore peg holes from the side orientation, not down from the top. I wonder if boring mortice holes from the side would add any benefit? The chips would fall away. Maybe better control of the Hole Hawg?

I tried the mirror, at one time, the last time, the bit got sucked out the exit side and the chuck was pulled into the circle hole in the center of the mirror and it broke into two pieces. Now, I have a mirror in the shop to look into as an aid in extraction of the occasional wood chip.