I wemt to "Target" in school supplys and got a whole set of clear plastic squares etc.
It has a protractor that works great for finding 90 degrees. I place the flat side on the table behind the blade and adjust until it lines up. It is also great for adjusting to any angle (either right tilt or left tilt).
Besides the protractor, it has an 8" ruler, a 45/45/90 triangle, and a 30/60/90 triangle. They all come in handy for drawing patterns and other shop uses.
I think it costs less than $5.00 but, it has been awhile since I bought it and cant rember.
Rog