If you use 1 cc of plenum per FPE you want, you really can't go wrong.... If the gun is really efficient, you can go smaller.... If you are going for very low power for the caliber (which is generally very efficient) you can go smaller.... If you are installing an in-tube regulator, where increasing the plenum volume makes the main reservoir smaller, you are actually better to go smaller (as small as 1/2 cc per FPE) and increase the regulator setpoint to make up for it.... because at 1 cc per FPE your reservoir will be so small you will lose too many shots....Since you are using a separate 0.61 L CF tank, your reservoir will be that size regardless of how big you make your plenum.... so I would make it as large as is reasonable.... Are you planning on shooting pellets (JSBs are about 81 gr.).... or bullets?.... If bullets, how much power are you after?.... You don't say if your CF tank is 3000 psi or 4500 psi.... If it's a 3000 psi fill, and you are regulating at about 2000 psi, you are very unlikely to be building a practical bullet shooter, IMO.... Even with a 28" barrel, 2000 psi would be hard pressed to get enough power to make a strong bullet shooter.... With 7mm (9/32") ports, which is not unusual for a .357/9mm.... you will be very lucky to hit 160 FPE (950 fps) with the 81 gr. JSBs with a 28" barrel at 2000 psi.... You should design for about a 150 cc minimum plenum to achieve that power....Bob