As I said, you need 3.8 psi oxygen per atm.
Your propane meter won't work for the GEN III because of the higher O2 content per atmosphere. If you want to use that meter, you will have to use fractional mixes which will be uber difficult to calculate.
If your reg tops out at 15 psi, then you will be limited to less than a 4x mix, so I would highly suggest either modifying the existing regulator (I modded a bernzomatic O2 reg to put out over 60 psi), or buying a whole new reg, which is safer, more reliable, and costs a s***load of money.
