As reference, when I go offroading, I air down my tires from around 28 psi to 13 psi. This number came from a lot of guess and check to see what pressure worked best for me.
Obviously, these 6" wheels (my team is also using these) are much smaller, so the difference in contact area when airing down will be drastically different, but I do have some neat reference pictures.
My team hasn't tested this yet as we are waiting to see how heavy the total robot will be. My suggestion, put your robot on the rock wall so the front wheels are on the edge of the wall, then start letting air out until the edge pushes into the wheel so you have contact on the vertical and horizontal face of the wall. This is obviously just an example and you can use this idea on any obstacle, but this is how we plan on testing ours.