Well bluenun, even though the exact average woman has a total canal of 6” when including the vulva, our fat pads take up some space. Also, the distance between your body and her body must be accounted for. I’d say that 7.5” bone pressed, with .5” to 1” fat pad would be reaching the posterior fornix well. The experiences of different men here can justify this. Also, 8” bpel seems to give a man more insurance that he will reach it in more than one position. So, the common goal of 8” bpel that is touted here has much physical evidence supporting it. I bet that 7” bpel would reach it with two very lean flexible lovers and an average vaginal depth. Life happens though, and many women are not average, and not all men and woman are lean.