Ok, I would start at the F3 and see where things measure to.
I am getting very strong base response at my seat below the F3 point, so I actually cut things a little lower.
In terms of understanding how to quickly and effectively use the TacT preamp, go to Yahoo and join the TacT user forums. Once you are a member, you can download guides put together by TacT users that run circles around the TacT documentation.
It can be a little difficult at first, but stick with it as the results when properly setup will amaze you!
George