It is best done by trial and error using the gauge on the pump as there are a lot of variables depending on calibre and if it is the H.E. type. I just did a Sirocco Deluxe and started off at 25 BAR, about 350p.s.i. Start with that then try cocking the rifle, if you have in too much it will be hard to cock. To drop pressure tap the valve very lightly with a punch and hammer to release some air and try to cock it again. If it cocks check the velocity over a chrony to see what the fps is. If too high tap the valve again, if too low put another pump in. It can take two or three goes to get right. The Sirocco is a pain as you have to keep putting it in the stock and taking it out to fill as the valve is below the level of the wood at the back.
Baz
BE AN INDEPENDENT THINKER, DON'T FOLLOW THE CROWD