We have a pub, and we really need to keep a close watch on profit margins. I
have a formula for calculating the GP from nett cost and selling prices [
which is 1-(cost price/sales price) ] but my maths lessons are nearly 50
years back, and I can't work out how to calculate the price we need to charge
in order to make the right margins.

A younger memory will solve this in seconds, I know, so please lend a hand.