Sounds a lot like the house I bowl in. Our house shot is 41' and I'd st on the heavy side. I've got my Tribal laid out pin under the bridge, cg kicked 45*. With this layout the ball gives me great length and a nice hard arcing backend motion. Almost snappy when its fresh, but still enough energy to carry once the carry down starts acting up.
Honestly, I would say trust your driller. He knows the shot I'm sure and more importantly, can watch you bowl and knows your game. That's the most important part. Usually when I suggest something for a customer I already have an idea of what kind of layout they'll need on it.
One more thing, you won't regret going with Motiv. I love my Tribal. Great piece. I've also got a Venom Toxin, Primal Rage, and Raptor Talon. I have yet to be disappointed with a Motiv. Awesome products.
Matt