I found a great way to check your miter extremely precisely for square. It made my head hurt adjusting it over and over, and I’ve come to the conclusion that I may never get it perfect, but it will be close enough that I can’t tell. This is from the woodbutcher.net site. He references Dave Lers’ page on the 32mm cabinet making system (http://32mm.dalrun.com/) I’ don’t know anything 32mm other than it is a cool way to check for square. Yes, I stuck my precision squares on the miter and the blade and it looked good to me but I guess I was off by a few thousandths!
The gyst of the story is that you take a relatively square piece of wood (bigger the better) with at least one straight edge and you makes cuts on each face working in a counter clockwise fashion around the board. After the 5th cut, you measure the sliver of wood that was removed. I can’t do it justice, but if the piece is bigger on the bottom (end that touched the miter) than the angle between the miter and the blade is over 90 degrees, it the bottom is smaller, the angle is smaller than 90 degrees. If the difference between the top and bottom is 1/32” it equates to about .001” difference in the diagonals. YIKES, that is precision!
Anyway, here’s the link to the link: