Well, the power valve is for enrichment of the power circuit. Hence its name. WHEN do you want that enrichment? Holley says cut the idle vacuum in half and choose the closest valve to that half vacuum reading. Why would you want enrichment to the power circuit at such a low vacuum? It's the power circuit. I get a vacuum gauge connected to manifold vacuum with a hose long enough to get the gauge inside so I or my passenger can read it at cruise speed. I get the cruise vacuum reading. In other words highway speed with barely enough gas to keep it t speed. The vacuum reading will be highest then. I take "whatever" that reading is, and choose a power valve a couple of numbers lower. That way, the power valve is actually opening when it's time for power circuit enrichment. When you get it about right, you can feel when the power valve opens. After tons of reading on forums and magazines and such, I tried experimenting with it and I found that that's the way that seems to work best for ME. You will also be able to drop the main primary jet sizes down some, since the power valve is now opening during cruise enrichment and that can even pick the mileage up some.