Re: Scope almost out of adjustment?
A Viper PST is a 75MOA elevation scope. So if you put a "flat" base on it, you theoretically have 37.5 MOA in the "up" direction. If you use a 30MOA tilted rail on it, that would allow 37.5+30 = 67.5MOA of "up" travel. If you absolutely have to get the most travel possible out of the scope then that is what it would take.
The downside of course is that one is not looking through the center of the optical system at either close range or at maximum range, but on the other hand, this is what these scopes are designed for.
I'm a little curious how you only end up with 20MOA of travel after zeroing ? "Using" 17.5MOA to zero a scope indicates that something is wrong. Are you sure that you don't have a 20MOA rail and have it on backwards ? That way you "used" 3MOA to zero it after an initial 20MOA correction (the wrong way). I would check if it is possible that the rail could be reversed.
OK, checked what it is that you have and the base is integrated into the ring. Is the front and back ring the same ? Very little info on the Talley website, but they say nothing about the one piece rings being inclined.