Mr. Griggs the purpose of tilting the hives forward is for water run off, so water, rain, will not stand in the back of a hive with solid bottom board.
So tilt enough that water will run out the entrance and not into the hive. Starting with a level hive, then raise the back of the hive one half an inch, 1/2 inch should do it.
Again, those are beautiful hives, stand, slab and type of wood, just beautiful. Makes my plain Jane hives look average, not exactly a high end neighborhood at my apiary but very functional here.
Blessings