I would point each item individually, add them together and weight the cake to find out the weight for all the PPs, then portion the cake into weighed slices so you know how much is in each..? Does that make sense?
2x bananas - 0pp
6oz SR flour - 17pp
6oz stork margarine - 25pp approx?
6oz sugar - 17pp
3 eggs - 6pp
65pp per cake. So if you split it into 8 EQUAL portions it would = 8pp per slice. But you would need to weigh the whole cake and then divide into 8 equal slices according to the weight....