So... are the tolerances that good on the home-center bar stock?
Personally, I
have my doubts, after having purchased 3/4" aluminum angle from HD that
turned out to be almost 0.020" out of straight over 24 inches. It's hard
for
me to imagine that the bar stock is an order of magnitude better.

... er actually it might well be. Al angle is extruded ... the extrusion
process often leads to a fair amount of bow. Steel bar stock is usually
cold-rolled and the straightness is pretty good.


Can't speak for Home Depot bar stock, but I've got an old Ryerson book in front
of me that gives their width tolerance for cold finished low carbon steel bars
as +0, -.003" for .750" and under. Since that's their tolerance, most of their
stock is better than that. No straightness tolerances specified.

How much do you think your miter slot varies from end to end?

John Martin