It seems to me that you would want your ELBOWS just below your hand height, and your hands at nipple level. That seems to be the place where it isolates the pecs the most. I think the most important thing is to keep "hugging the tree", in other words dont allow your elbows to hinge during the "press" Start with the tree hug in place, and just use the pecs alone throughout the exercize....
Does that make sense to you?