Learned this from Augie, a press tech at M&R and all around cool guy. Check it:
So basically what you do is add whatever spacer you want under the level on one end.
If you want you can tape the spacer to the level.
Then the other end of the level goes on the frame holder. You adjust the frame holder till it's "Level".
Do this to all 4 of the frame holder corners and you should be good to go.
This might actually be better with a digital level.