Leveling subfloor - 1/4 ply?

Is there any problem using 1/4" ply to build up an existing plank wood subfloor? The planks are 7-1/2"w x 3/4" set at 45 degrees diagonal to the joists. I want to use 1/4" to make the new floor level with flooring in the adjacent room i.e. want a level transition. Will the thin 1/4" ply create any unforeseen issues, even though I'm relying on it only for leveling? If its Kosher, then how would you attach the ply? 1" fully threaded screws every 6" on the edge and 8-10" in the field? btw: * the new hardwood (going over the planks+1/4 ply) is 3/4" solid hickory. * after reading other posts on the subject of sub-floors, I've decided NOT to use glue between the ply and planks, and NOT to nail the new ply through to the joists.