With carb heat, always, always keep the inlet temperature out of the yellow. I always fly with +10.
And be aware that the "temperature assist" doesn't always do you favours. As you transition and reduce power, the carb heat knob actually drops. If you are doing training across the airfield, you'll more than likely be constantly starting and stopping exercises frequently. Make sure the knob is always up at least an inch.
I'm not a great fan of the temp assist mechanism - it never seems to assist when you need it and its reactions in no way reflect the actual inlet temperature.