FX HOW DO YOU LOWER IMPACT SPEED?

I am reading about people achieving better results shooting slugs at lower speeds with their Impacts.

Just quick question…

Do you simply lower speed with the power wheel or do you retune down to a lower speed with the reg and hammer adjust screw?

Right now I’m shooting .25 slugs at around 970 fps on PW=12. Groups aren’t bad; could be better.

If I wanted to try say 900 fps is it a simple turn down to a lower power wheel setting or is it better to reset the tune to 900?
Hoping I’m being clear on this.
 
Last week I was re-tuning - from last year slugs (960 ish) to next season pellets (880 ish). What I found so far that turning down the Power Wheel alone didn't gave me tightest groups how I want it or where I was imagine to get there. Next time in my club I will need to dedicate more time and instead of a PW start with lowering both the Reg and PW in one big step and start again somewhere from 130 b and raising incrementally with small steps.
 
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