Please forgive what may be a dim question but I am new to this whole concept (although I must have had the bug for a while because I once chose to fly from Darwin to MEL via Groote Eylandt and Cairns partly for the points) - but when you are calculating the cost do you use the cost of the whole fare on the mileage earning ticket or the difference between the cost of the mileage earning one and what you could have got it for on a low cost carrier?