Using the 6in. machinists rule is a trick I learned long ago. I use that method all the time and is the best lathe tip there is. To this day I still amaze veteran machinists regularly who had never even heard of it.
Idea to try. Set a combination square on your compound to compare the vertical tilt of your pinched scale. By having a fixed vertical reference you can easily tell which way the scale is tilting (leaning).