IDK about that. I’m 6.75 BPEL and maybe 6 EL, and yes I go balls deep, stroke her cervix, but if I was an inch longer she’d be in pain. How do I know? From the 8 inch dildos I’ve fucked her with and she didn’t like.
There is great debate whether or not there is much usable penis beneath the fat pad. I've heard arguments from both sides. My best guess is that for some people, yes there is a fair amount of usable penis and for others maybe not. Everybody has different anatomy.
People say, "I can grind bone to bone with my woman, so I know there is usable penis," or, "try sticking your dick into a glory hole and tell me you can't use all of the penis bone pressed."
In my case and i think for most you can use most of your BP length. My best guess is I can use everything but the last .25" or so. Im just over 7" BP and can get into my wife's deep spot on most occasions so I think thats a fair estimate. Plus my fat pad is easily depressed with my fingers revealing near 7" of visible dick.