You are using 30/70 mixed gas (the "convention" is to state the CO2 first ... but no-one does!). This is going to get messy! "Partial Pressure", "Absolute pressure", blah, blah, blah. You'd be better off using pure CO2 and the LPG regulators I use (as a secondary regulator - they can't handle cylinder pressure). But you want mixed gas? So:
Forget the carbonation calculators, they can't do this! For 1.2 volumes CO2 you need about 2.5-3PSIG (ambient temperature, "G" for what an ordinary gauge tells you.). Add 15PSI to work with it at absolute pressure ... 18PSI ("absolute", no "G"!). Divide by 3 and multiply be 10 to account for "Partial Pressure" ... 60PSI. Subtract 15 to get it back to 45PSIG. Blimey, that's a lot! That's at ambient temperature remember. If you can get down to single digit temperatures, you should get the pressure down to 30-35PSIG.
Now: Forget it and get 100% CO2.