I have possibly the same springs on my F250.
With 2 more ply added to my rear I go down just enough not to be supported by the bottom plate and the helpers are just touching,
Before adding the second ply I was supporting to heavy on the bottom plate and the ride was a little stiff so I added the additional short ply to keep the springs away from the plate. This setup was on the truck when I bought it and pulled the previous lighter trailer with it and the ride was OK. But I though the ride would be better with the short ply removed, but then the BC made the springs ride on the bottom plate and I figured no improvement on the ride.
Just mesured the drop at 2.5" and the truck now rides perfectly level. With the one additional ply it was riding 3" low and the truck realy looked good also.