watts / volts=amps.
amps x volts=watts.
The problem is with power supplies with 2 or more 12v rails.Where each one is rated at 17amps. 17 +17=34. but the maximum combined for the 2 is 27. so if one is pulling the max of 17 the other will be limited to 10.
Yes its true, and easier for single rails. If a single rail PSU can output 360W on the 12V rail, then 360W / 12V = 30A. The math is clear and simple. Its the multi rail PSUs that are problematic as you need to find the total wattage putput, and not all PSUs make this easy.
No, 70A. It obviously can't output all 70A if you are using a lot of 5V or 3.3V devices. As I said they are using DC to DC to give you the minor rails. Its a single rail, so the total possible output is 70A.