@opfor congratulations on your purchase and welcome to the forum. I think the avenge x provides a lot of value for the buck and a good entry into PCP shooting. To answer your other question, a sweet spot for diablo pellets is generally regarded to be around 875 FPS for accuracy. Yes you will need the chronograph to verify that.
@Ray_PR seems to have his dialed in nicely at that velocity so his data might be a good model to follow.
You can try dialing down your hammer energy in increments and seeing if you're groups change until you get your chronograph. You may notice a reduction in the discharge sound and efficiency / shots per fill. Faster velocities are not always better. I like 875 FPS in my better PCPs that can achieve it. The pellet exits the barrel so quickly that it's less susceptible to movement during the shot cycle than with lower velocities. Of course, follow through for the shot is always recommended in any case.
Very smart to keep a notebook of original settings and write down your adjustments so that you can keep track and not get too far away from a good setup.
A good barrel swabbing will help reduce your flyers. At 50 yd, it doesn't take much wind to produce them or a tick in your posture during the shot cycle.
I would remove the moderator and inspect the crown very carefully for any machining burrs or manufacturing gunk. If possible on this design, push a pellet through the barrel with a dowel and inspect it for any unusual marks other than from good smooth rifling that might tell you if there is any burrs in the breach or transfer port area that you can touch up.
Have fun and enjoy PCP shooting!
Feinwerk