A3977 Calculating ITrip

I'm missing a vital bit of information about setting up an A3977
based unit. The A3977 explains how to set ITrip (the current at
which you want to cut off the motor supply) but I can't seem to find
anything that specifically says how to determine what ITrip value
you'd want to use.

Lets say I use a 5V 1A motors. I'm going to be supplying them with
20 volts which means the maximum current I want to allow is .25A.
Is .25A where I want to set ITrip? Seems a tad high, supplying
maximum current all the time...

Is there a formula to determine the ideal current or even just
a "increase until X happens" method?

Thanks in advance!