Once you have the trailer level side to side, next comes front to back. My home driveway is slanted to the side and front to back. I use that yellow triple level block under the trailer's driver's side tire. Then I use heavy duty blocks on all tires of the trailer. The trailer is then disconnected from the truck and I level my trailer front to back. I place a two foot level on the top of the A-Frame and use my power jack to lift the whole trailer up. It has to go up in the air so much, that I place a 6X6X18 inch block under the bottom lift plate before I start! Then I get the trailer level front to back based on the bubble reading of the level.
Finally, I take a bunch of 4X4x18 inch wood blocks and start stacking them up. Place down two blocks parallel to each other, and about 10 inches apart; then two more 90 degrees on top of those bottom blocks working your way up. The goal is to be able to bring down the scissor jacks with about 50% left for more stabilizing scissor action. I use the 18volt battery drill to bring the scissor jacks up and down. I have used this method countless times on just about every conceivable parking spot I have been at and it has worked fast and accurate each time. I did purchase a similar set of blocks like you have pictured above with the yellow rope handles. I have never used them yet for stabilizing the scissor jack locations.
Anyway, that's my story and I am sticking to it
. Happy camping!