That circuit doesn't stop th motor, it just outputs a signal when the current limit is exceeded.
The transistor starts to turn on when the base emitter voltage exceeds about 0.6V to 0.7V so the current limit can be calculated using Ohm's law, 0.6/0.8 = 0.75R.
If you want to shut down the motor, you'll need to have a time delay.
Why not just use a polyfuse as I said in my response on the last page?
I know it isn't the done thing, but you'll need to select a polyswitch with a trip current of about 80% of the desired value, otherwise it's possible it'll never trip.