Set your rifle with correctly adjusted for you using a bipod/rear support of some kind on a flat surface as level (front to rear) as you can get it. Measure the distance between the bottom of the buttstock at the bipod attachment point to the surface. This will give you the height required to for shooting perfectly level (no cant). Next, decide what the location of where you do the majority of your shooting dictates with regard to upward or downward cant. For example, some ranges dictate that you will always be aiming downhill to a greater or lesser degree, some are the opposite. Then go to the Accushot website and determine from the non-canted starting distance you measured and the expected cant (up, down, or level)you will have most often, then pick the length that will give you the most adjustment within those parameters.
Accu-shot has pretty much the same guidelines for selecting the proper monopod length on their website. You probably want to take a look there as well. Good luck.