Its fairly simple maths really. Just have a calculator (or an iphone) at hand.
If I see an item being sold on the TP for say 6s 35c I just do this simple calculation:-
635 × 0.85 = 539
This auto takes off the 15% you will be charged by the TP
This tells me I need to buy this item at 539 to break even. Buying at anything above this figure is at a loss and buying under this figure will be profit.