The Omega scope is set up for a 250gr bullet launched with 150gr of powder which means somewhere in the 2100 to 2200fps velocity range. A rough estimate of drop puts the bullet down about 18 to 21 inches at 250 yards so that gives us an idea of the holdover for the 250 aiming circle. If you have an accurate load that is going say 1800fps it will drop 20 inches at about 225 yards using the 250gr bullet. Again, these are rough estimates and each bullet will fly different based on its own characteristics.

My suggestion would be to first find an accurate load then sight in the scope at 100 yards. After your sighted in, put an aiming dot at the bottom of your target and then shoot a 3 shot group using each of the aiming circles of the reticle at the same 100 yard target using the refrence dot at the bottom for point of aim. Measure each groups height above the dot to get an idea of the difference between POA and POI. Then look at a trajectory chart for your bullet and see what velocity matches the actual results of shooting, remember the measurements you got from the target will be reversed (drop) on the trajectory table. This is not real precise but will get you fairly close at which point you can start practicing at the different ranges you come up with to confirm and fine tune.

BTW, my son's optima likes the 250gr shockwaves with anywhere from 80 to 100gr of 777. I also used the shockwave for the estimates above.