It's worth mentioning: that the venturi can reduce the temperature by up to 25 degrees centigrade, also that carb. heat enriches the mixture and therefore leaning is important when carb. ht. is to be applied for long periods.
Opened throttle to turn and backtrack. Engine spluttered.
The argument as to whether the carb heat should be maintained until touch down or not is an old chestnut. As many failures can be caused by the carb. ht. being left on too long and causing a rich cut instead. This can be particularly so during touch and go and a go around, and especially on hot days. Remember that when the throttle is increased an accelerator pump gives an extra squirt to minimize take up lag. I think that 300ft is too soon to go to cold air but below 200 ft is a good compromise. I suspect that 300 ft is rollover to do with multi-engine 'committal' heights than carb icing considerations.