Hooligan1,
Read
this article.
As far as the length of the bullet, I don't know the weather conditions you have been testing under, but in standard conditions a 9.5" barrel should be able to stabilize your 150 just fine. The only hitch may be that it isn't fully recovered from initial yaw at 100 yards. You might check group size at 200 if you can.
As far as seating depth goes, some throats want the bullet up close, while others like it back further.
This thread's first post is Berger's method of finding the desired depth.
Try
Dan Newberry's method of identifying your sweet spot load. Work up in .7% steps, or about .5 grains for the powders you are using.