The calculations for feeders are 100% of noncontinuous loads + 125% of continuous loads. So the 24A times 1.25 is 30 + 12 = 42. That would need at least a 42A supply to the subpanel, eh?With a 30A circuit, the car draws 24A (or 20A if you dial it down in the car). So there is 6A (30-24) at 240V available on the feeder circuit from the main panel. Which is 12A at 120V.
Normally, I wouldn't trust myself to do a more complicated load calculation on a subpanel if it is considered living space, and it has several circuits, where you have to apply some amount of watts per square foot and all of that, but this one is really simple because the 24A continuous supply to an EVSE really does consume the entirety of a 30A supply circuit, so there is zero capacity leftover for anything else at all.