Ruger Precision 20 moa mount?

[killemall419]

I have a Ruger precision rifle and after I zeroed my scope I was checking to see how much travel I had left in upward travel to determine how far I could dial. I thought since the scope I'm using (Burris xtr II 5-25) has 90 moa total travel, I would have somewhere close to 65 moa of dial up with the 20 moa base on the RPR (the 45 from half of 90 plus the 20 in the rail). However, when I checked, the scope only had a total of 54 1/2 moa of up travel available. Could someone help me understand why the math isn't adding up or maybe what I did wrong?