Eckhard thanks for that.
So an example;
Sywell Aerdrome has a elevation of 424ft which ÷ 30ft = 14.13mb which means we add 14mb (rounded) to the subscale to give us QNH.
To get the QFE we do 424ft ÷ 30ft = 14.13mb which means we take 14mb (rounded) off the subscale to give us QFE.
Is that correct?