I maxxed out at 47 today without taking any kind of rest at all -- that includes sitting stationary at the top and bottom of my form. That was with someone there telling me if I missed full ROM, lost form in rigid back, "snaked" up, etc...all of which I consider cheating and not a true, honest pushup.
As for rest, I try to keep it rhythmic...same time to go up and down on #1 as the last one. When I deviate from that, I know my max is very close.
If I let my form suffer I could probably do much more...
Form is crucial in the pushup because any deviation from good form usually makes the moment much easier....