If you can make your cuts EXACT, there will be no overlap/excess/loss. But it has to be EXACT. 4- 5x12's and 2- 4x10's. Imagine your 16x20 in landscape position. You place 4 5x12's in portrait across the top. That spans the twenty inches. Place 2- 4x10's in landscape across the bottom, end to end and this takes up the four inches left over vertically and spans, again, 20 inches.