Once you get rid of the slop between the saw and rail, as already stated, the main issue is to check for true 0 and 45. Try to use as thick a stock as possible for the test. On the 0, rip a board down it's length, and then flip it 180 degrees and butt it against the other board on the cut side and see if there are any gaps in the joint. Do same thing for 45, but use a square you trust to test if the resulting mating pieces give you a 90 degree angle.