first, benches that I install, I create an intention "fall" to the bench so that water will not pond on the bench. A slight fall toward the drain will flow the water off the bench.
Now, I would do this, and maybe you have rebuilt this beyond my suggestions, thats fine.
First, I would not install any pressed wood at all, I would use CCA plywood in 3/4 inch. Then install cementious backer board over this. Now, I would completely reconstruct the bench to obtain level with the forward fall. I would not trust any levelor that is intended to bridge that much of a off level amount. It just will not work. You can work with tile on a forward fall, but not a sideways fall.
make it right this time so that you have a nice level and smooth bench.