AirGun Tuning

I need a little help understanding the whole turning issue. 

I have a Air Venturi Avenger.22 and I still don't understand the tuning.

I want to achieve a high shot count say 70-90 shoots per fill

Currently I have the reg set at 1400psi and 1 1/2 Hammer springs turned in

I only get 40-50 good shots.

My question is , is this the best I can hope for in my gun ?

If I have the reg down low as I can go and Hs turned to zero , shouldn't that be the max shoot count?

I have heard about the so called sweet spot of a gun but is't that just the accuracy and speed at which the gun shoots? Not necessary max shoots