I have an absolute minimum of 300 psi to get on board with. This covers gage error of up to 10% with me being onthe surface. If no error i still have gas to breath till my feet gets out of the water. You reg doea not want to work so well when secondary pressure gets below 150 psi. It cant supply air fast enough to breath easily. So now add the 300 psi to the 150 psi and you get 450 psi. This is tank specifics out of the picture. Now you have to add to that how much gas you need in cu ft to get from bottom to the surface with a buddy on your tank. (Rock bottom) . If that is another 400 psi for your tank then then you start your ascent based on your tank at 850psi. Your buddy may have say an lp120 on him and his start to surface psi may be 600. Between the 2 of you you start up at the highest PSI because the highest psi is the smallest tank.