I've noticed a lot of variation in cheap pellets. My Gamo Raptors (gold ones) are 5.15, yet advertised at 5.4. Lead pellets are even worse, with Gamo and Daisy being the biggest offenders imo, along with any chinese made pellets. Like some Daisy that I bought vary from 7.1 to 7.9 from batch to batch,so you dial your gun in one one tin, then you have to do it all over for the next. Better brands like H&N, JSB, RWS are much nicer, and Crosman "Premier" are consistent too. I prefer the JSB round nose, and for that gun I'd use a minimum weight of 8.44gr, but the 10.3 might be better. They also make a 13.4 you might like, but as pellet weight goes up you lose some power, which varies from gun to gun. The 8.4 should still have peak power, yet has some weight to it. Lighter pellets are harder on the gun, mostly harder on the scope since the scope is the weak link. So as a general rule the heavier the better for accuracy, gun smoothness, scope life, and they're quieter, but power peaks ~8.4gr so while 10+ will be nicer you'll lose some power and of course the trajectory is more arched. The heavies however work better for longer ranges because they hold what power they do have longer, so while an 8g might come out 10% more power (ftlbs), at some point, maybe 30-50 yards the 8gr will have lost so much than it's now behind a 10gr in power. Google Chairgun which is an excellent program for learning about pellet ballistics.
You'll need to know the BC of your pellet, its weight and velocity. One nice thing about JSB is they have excellent ballistics so even the 8.4gr will probably be better than other brands 10+gr. Daisy btw generally have terrible ballistics so they are imo lose lose pellets. JSB with their excellent ballistics are a win win with their precision and outstanding BC. I'd order a tin of 8.4 and 10.3 round nose JSB, I'm sure you'll like them.