I also shoot a Black Diamond XR. I am shooting the T/C Shockwave 250 grain .44 caliber projectile with 100 grains of Pyrodex. According to the charts, a 250 grain SST/ML (which are similar to the ones you are shooting) being pushed by 2 pyrodex pellets on that projectile, the Muzzle Velocity (fps) would be around 1890 and still around 1284 at 200 yards. The Energy (ft-lbs) at the muzzle show 1983 and still around 925 (ft-lbs) at 200 yards which is well above the 800 (ft-lbs) many people claim necessary to make a clean kill.

I am also working up a load with the 300 grain .45 caliber Hornady XTP's but they load very hard in the rifle. Another projectile that loads nice and shoots fantastic are the Precision Bullet .45 caliber 300 grain Silver Lightening Poly Tip (green tips out of a black high pressure Harvester Sabot). Their only draw back is the cost.

So if I were you, I would go ahead and shoot the 90 grains. This load should be fine out to even 150 yards. Bullet placement is most important as we all know, so use the most accurate load. Hope this helped