Technical question about regulators

Hi all !!

I was wondering... What really matters with the reg, is the pressure which is applied to each shot and not the volume of air right?
I mean, the reg regulates the pressure and the combination Hammer / Hammer spring / valve sets the volume of air for each shot right ?

So... With a 48 centimeters long barrel on a .25 cal rifle, with 4,5 millimeters transfer port and barrel port, a reg set at 135 bars and the correct adjustments of Hammer and Hammer spring to achieve 900 fps with 25,4 gr jsb... What would be the ideal volume of the plenum / regulated air ??

I'm assuming here:
If each 3,3 cc of air (at a 135 bars pressure) is needed for a shot of .25 at 900 fps, but your plenum is 16 cc, then there is still 12,7 cc of regulated air that is not used right??

I'm getting the synergy of the whole firing process but I struggle with getting the initial data for each part of the process (regulated volume of air needed so volume of the plenum needed, minimum transfer port size needed for this kind of power (65 joules) in .25 cal out of a 48 cm barrel etc etc..)

I'm asking this as I've ordered a Kral puncher Breaker with a 48 cm CZ choked barrel and the gunsmith is telling me we can't push it up to more than 55 joules. Still, I'm sure that with a bit of work on the transfer port and barrel port... We can easily go up to 65 joules...