Help needed with automatic formula population

Thank you for your help with this John.

your formula works exactly right for week 1 but only for 70% of week 2 (that's probably down to my crap formula in week 1 though)

i have entered the original formulas into week 2 now and what the results should be and then done a copy using your formula and highlighted which cells are incorrect - i cant seem to adapt it to work as i'm not 100% sure on what it does

could you kindly point me in the right direction please.