I have an application that currently uses the following formula for calculating the margin on an item sold:

lnMargin = ROUND((1-(tnCost/tnPrice)) * 100,1)

The problem is that we have customers entering tncost > tnPrice to create a negative margin/loss leader situation. In this case, the returns from the function are huge and cause an overflow.

Can someone tell me or show me a formula that will calculate negative margins correctly?

Thanks..