You may want to wait and let the gun choose the pellets. All of my .177 HWs shoot the 7.9gr Crosman Lites better and more consistently than JSBs. Yours may differ, but don't count on one pellet before you shoot the gun to see what's most accurate. Personally I stay nearer the 8gr range with guns at that power level. Heavier pellets tend to cause piston bounce and that can increase hold sensitivity.
I sight in at 30-35 yards on my HW77K rifle for field target out to 55 yards. I use the same gun or similar guns for hunting, so I do the same for those. Mine run around 890fps with 7.9gr pellets.
As always, your mileage may vary so try a few things and see what works best for you. Either way, you picked a nice gun and you'll enjoy it!