I doubt that the Nikon Monarch 3x9 is calculated for a 50 cal ML. However, the Nikon "Omega" 3x9 scope is. The Omega scope may also be in the Monarch series, I can't remember. The Omega was originally designed with T/C's "Omega" ML in mind which is capable of handling 150gr volumetric equivelent charges of propellant.
The BDC circles should represent the drop of a 250gr projectile that is propelled by 150gr of Pyrodex (or similar powder). With the crosshair @ 100yds the circles should represent 150yd - 200yds - 225yds - 250 yds.
I bought one to try out but haven't mounted it yet on a new Encore ProHunter. Other shooters that have this same set-up tell me that the circles are "on the money" for hold-over providing that you are using the designated load. I'd be interested to see how much difference a 300gr spire bullet would make. I'd assume that yopu could rotate the power ring slightly and bring the 250yd circle back into play regardless of bullet weight or speed. Within reason, anyway.