Hammer springs FAC

[angrybear]

A question for tech/tuning experts;

I'm in the process of changing my FAC .25 mk2 from FF to regulated, hoping to improve velocity &/or shot count.
I've swapped barrels to go from a 3.8mm port to 5mm, & after much faffing have got a short core reg running at 150bar, & a Dr bob hi-flow valve.
But I'm having an issue with hammer springs 

I believe a std spring is 72mm, mine is cut down to 67mm, if I use a 5mm top hat to extend the length, on shooting the valve does not close (too much preload) I have shorter heavier springs, or longer lighter springs or a range around the same length including a progressive,
to save me having to try them all which is likely to give best results.

FF currently does about 884 fps (44ftlb) with JSB 25.4gn & gives 52ish full power shots from 220bar fill 400cc bottle.
I'd like to get 900+fps, in my dreams 950fps for 50ftlb.