If your only shooting 500 to 600 yards only I dont feel you need a + 20MOA base mount. What it really does, lets say for 1000 yard shoother, is to not have to adjust their scopes to the max of their internal adjuxtment MOA range. Know the MOA range adjustment of your scope and the ballisstics of your bullet before making a decision to use a +20 or +15 MOA base mount before you buy. When you exceede the 600 yd. range for targets the + MOA base mounts can come into play for a reasonable solution for your endevor, however, less yardage than that I dont feel you need it, unless your scope is a POS. for the application.
Also, when your shooting this disatnce (500 yards+) you shouldn't be scrimping of cost of glass (scope)