For example if an agent wants a 10% rebate we need to calculate the markup from our standard selling price to accommodate this. If we sell a product for £100 then we'd need to invoice it for £111.11 because 10% of that brings our real selling price back to £100.

If we mark it up by 10% we'd invoice out at £110, the 10% rebate to the agent would be £11 and we'd have sold for only £99.

Comprehending that you don't use the same percentage up and back down is too much for most people here. They eventually get to understand that they can't do it that way, but actually doing it correctly is a dark art.