Circular reference issue. Looking for workaround.

I have 3 columns (Quantity, Rate, Charges). If the quantity is 0, I want the Rate to equal the Charges. This is for an expense entry on our invoices. If we have an expense that is a flat fee, I want the quantity to be listed as 0, the Rate to be the amount of the flat fee and the Charges to be equal to the rate. That saves my staff from typing in the same amount twice. I used the following forumula, but get a circular reference error.

=IF(D1=0, E1=F1)

I tested it with $75 as the rate and I get a result of $0. If I tweak the iterations setting I get a result of FALSE. I'm stuck. It seems so simple, but I can't figure out how else to create the formula. Any help would be appreciated.

Beautiful! That worked. Thanks for your help!