He should really be looking at voltage drop if he is really using any appreciable part of that 70a. I assumed he was just planning on a lot of inrush on his motors. I do agree you can hold your nose and "round up". It might not even be a terrible design decision if the FLA of the motors is 15-20a, plus whatever he would have on a pair of 20a 120v circuits. The cruising amps may really be in the 25-30a range.
