It depends on the airfield in question. For example, one airfield that I know always uses QNH and the circuit altitude is published as 1200'. As the airfield elevation is approx 200' this represents a circuit height of 1000' above ground level. Another I know uses QFE, but it is only around 50' above sea level. I suppose that that must be the reason for the choice between the two.
Your man with 300' on the clock should have been using the local i.e. airfield's QNH, not the regional setting.