Grumpy,
I have used in the past a straight edge with a bolt and a nut welded to it. What I did was put the straight edge on the rear roller and front roller and then adjust the bolt until it was just touching th bottom blade on one side.
Then I would move it across to the other side to see it had any difference and if you did you knew that the frame was either twisted or the bottom blade was bent or the height adjustment was out.
