I think the calculations should be like this: if you can get to 15m from the surface on a mouthfill then the presure would be 2.5atm = 2.5x the presure at which you did the mouthfill. So lets say you manage a mouthfill at 20m (3atm) it would take you to (2.5 x 3 = 7.5atm) 65m and if you stretch your lung so you can make the mouthfill deeper and practise the mouthfill you sould be able to get much deeper...(if you manage to do the mouthfill at 25m you'll get to 78m)
I think...
/Ulf
