Using vkp5 to calculate purch. price based on sales price

[Rudy Zijp]

Hi,
My client works in a market where the sales prices are dictated by the government (yes, medicines). With some suppliers they have an agreement where they get a fixed margin.
Is there a way that I can tweek VKP5 so that I can calculate the purchasing based on the sales price and a fixed margin?
Kind regards,
Rudy