Understanding MOSFET ouput characteristics on a log-log scale
You can project the slopes back to get a reasonable approximation. Or you can assume current is ten times higher at 50 A. For instance with a gate voltage of 6V you'll get this: -



Volt drop will be 0.43 V at 50A so just assume a straight line and at 5 A you will see a volt drop of about 43 mV = 215 mW dissipation.

MOSFETs like this have a Rds(on) value dictated by Vgs in the "linear region" and it's a fairly good approximation to assume Rds is also linear in this region.