fishy68,
The reason that this doesnt work is the fact that flow benches differ from bench to bench, but when done this way, would be more equal from bench to bench. As some benches will flow more at high lifts and some will flow more at low lifts. But keeping the bore size and depression the same helps this too.
This is why when flow benching, what ever bench you start on you need to finish on as otherwise you start racing flow benches. The whole thing about flow benching is to maximize the ports for best performance, and see how much change you have made. If you use one bench and then another you wont know if the change is better or worse as the flows will be different. So it's important to use one operator and one bench when doing flow and porting work. This is why good head shops have ther own flow benches and trained personel that operate them, so results can be duplicated.