Joseph,
Did Vortex only label your turret to 900 or are you saying you're running out of adjustment at 1000? I'm just trying to get a better picture of exactly what you're trying to do. If you're running out of adjustment you could always get a 20 MOA base, but I doubt that's what you're saying. I would think that Vortex would label the turret for as far as you wanted.
But anyways, as mentioned before, I think if you got a real tall target and used a level to make a straight line vertical and horizontal, that would help you figure out exactly how many clicks you need. Because if you have no holes to show where you hit, then you have no way to improve, which I think is what you've already found out.
Scott