The Burris xtr rings are in a class of their own when it comes to measurements for height.Only real way to tell is go to their web site and look up the measurements. While there request a catalog and they will be printed along with the other ring heights for there brands.
I don't think they really have a low in the xtr I think it is = to a medium, the med.is a xtra high and the high is like xxhigh.
I made the mistake of assuming and was off by 2 sizes now I look up everything when I buy Burris rings,but for what its worth I think they make the best rings out there so it is worth the effort.