I wouldn't cut a 1:8 twist barrel below 8.5". Every bullet deserves a full turn in the barrel.
Meh, I stabilize 300+gr 44mag bullets in a 1:20 twist with only 17" of barrel.
Its not how long its on the tracks, but if its moving fast enough forward and rotating fast enough. More time in the rifling beyond the fist few inches really isn't going to affect stability, it's the speed and rate of twist that should be the focus.