Ignoramus3537 wrote:

When making that mounting plate, I realized that there is a lot to
precision drilling, and want to ask now if anyone knows tricks for
drilling precise holes. I have two more plates to make.

The required precision for locating encoder base is 0.01". (which is
not that bad).
Well, you need a CNC machine to do that! The problem is that without
accurate measuring of table travel, it is hard to do. You can use a
boring head to bore the central hole, and any centering boss or recess
at your (0,0) coordinate. but, then you need to spot and drill 4 holes
at the corners. A machine with worn Acme screws and no DRO will give
you fits as there is differential wear on the screws, so even relative
movements are wrong. (My old manual Bridgeport had this disease,
nothing would ever fit no matter how careful I was.)

Yoiu can try scribing marks with calipers, or even surface plates and
height gauges, but it all comes down to how accurate you can dial in the
drill bit over the mark. I was never very good at this.

Jon