I have cut 2 x 10 pressure treated lumber into 10" x 10" x 2" (which is really 1 1/2 " thick, but whatever.)
I have a plastic tote that holds 12 of these blocks. I put about 4 blocks under each of the front rams. I then put at least one block under each of the four remaining rams. I then use the plastic "lego" type blocks on top of each of the four rear blocks.
I figure most of the force will be on the front legs, so I did not want to use plastic under the front rams. I have three bundles of the plastic blocks so I will use at least four of the plastic blocks under each of the rear rams.
After I do this, I just let the auto level do its thing. I figure that the less amount of distance the rams have to travel to level, the better. This also makes for a more stable footprint.
Just my two cents worth.