If you're shooting for 45#, just pull it until your bow scale reads 45#, and see how far that is. If the limbs are traveling at least 10" when you pull it to 45#, go ahead and string it.
After that, just pull it to 45#, and tiller from there. Never pull it more than 45#. Keep removing wood from the belly, checking to make sure the tiller is even, until it's 45# at 27". Then you're done. After you sand it and shoot it a bit, it should settle in at 45# at 28".