power valve?

Hey, everybody. Got just a quick ? about holley power valves. I know you check your vacuum at idle and divide that in half to get the size for the valve- but the catch I have run across is do you check it in park or in drive? Tranny is 350th automatic. My vacuum in park is 14 in. and in drive it it 9 in. So do I get a valve of 4.5 or get a valve of 7 in? Thanks.

thank you, it's been a little rich but have been running the 6.5 that came in the carb to start with. Gonna go get one and see what happens.:rolleyes:

Warmed it up and checked in gear with idle about 6-650 rpm and got the reading of 9in. Off to get grungy:LOL: