How to calculate free wheeling current value
Since inductors work against changes in current. The maximum (peak) freewheeling current is 30A same as the load current.
When the switch opens this current will flow through the freewheel diode. internal resistance in the coil and the voltage drop of the diode will cause this voltage to drop fairly rapidly. 

One sure way to kill that FR607 is contact chatter, because it's only rated for 6A and a 50% duty cycle could put average 15A (or more) through it, so make sure you're not getting accidental PWM from the relay.

Perhaps use an automotive rectifier diode (like are used for altenators), depending on how often it is switched you may not need a heatsink