I never put two boxes of foundation on. I would take them both off and check the two bottom boxes. If there is less than 8 full frames in the two bottom boxes, I would put 10 empties in one box and remove it also. If there is more than 8, but less than 16 full frames, I would leave it with those 2 only. If there is more than 16 full frames, I would put 1 super back on.
Again, when 8 frames or more are full, add a box.