Reply by higtron

Take a piece of plywood that has two parallel edges put one of the parallel edges against the fence of the cross cut sled trim one end now flip the piece over keeping the same parallel edge against the fence now make a cross cut on the opposite end measure from corner to corner both directions on this piece if both edges ar equal the fence is square.

-- If I cut it too short I can scab a piece on, but if it's too long what do I do?