Your best bet is to go by the maximum point blank range. This is data that you can get from the ballistics information specific to your projectile. This distance in layman's terms, gives you the sight in distance in which your projectile will be "in the kill zone" for the maximum section of distance. If you give me your ballistics information, i.e.: caliber, bullet, and velocity, I would be more than happy to look it up for you in the ballistics charts.
For example: A .223 shooting a 55grain vmax, B.C. of .255 and a sectional density of .157 with a velocity of 3240 fps (std. factory hornady load)
MPBR is basically a measure of the maximum distance in which the bullet trajectory stays within a 6" window "kill zone"
So if you sight your gun with this setup in for dead nuts at ~260 yards, the trajectory of your bullet will be within the kill zone from 0 yards to 300 yards in which at 300 yards the bullet is below the sight line by 3.1".
Hope this helps.