Larry, you have to calculate effective voltage as:
battery no load voltage - current * ( resitance of battery + resistance of wires + resistence of ESC + resistance of winding )
I think your question is what is the voltage drop on ESC only ... so if the current is up to 40A, and resistance could be ~2mohm, then the voltage drop will be 40mV when fetts are open. If we run in PWM partial throttle, everything is different and depends on % of throttle because we have 1/ switching loses (the resistance is higher) and 2/ the average current in fetts is higher than average current in battery leads.
and an example, this page shows ESC resistance for Jeti ESC:
http://www.jetimodel.cz/index.php?page=product&id=42