Looking for best settings on an Imact .25

I second, third, fourth the statement not to tune using the gauges on the Impact. Ernest recently put up a video on "what not to do" when adjusting the regulator. He basically says not to run the adjust screw flush with the regulator body but rather from flush turn the screw back in one full turn in to achieve about 130 bar. I plan to do that with my gun when I get home from my trip in a few weeks. You can run it out a bit from the full turn in but don't go flush or you run the risk of blowing the o-rings. 

Ignore the FX gauge readings at that point. Mine are 30 bar off from bottle to regulator and the bottle reads higher than the fill tank's so disregard them. 

Watch his video:

http://www.airgunnation.com/topic/fx-impact-reg-adjustment-limitation-what-not-to-do-prevent-damage-orings/

I think that you'll be able to achieve 860 fps with EK Heavies. 

I also have very good luck with the 25.4 grain Kings and I chose to start at 888 fps, because thats where they did well in my Cricket. I think they are good from that speed on up to 920. You're gun may be different so experiment.

I've been shooting Heavies on Setting 5 and Kings on setting C with the Power wheel. Thought I'd mess with the alphabet side to see how that goes. Works good.

Good luck.