I have a purchase order database, where products are selected using value lists and after making your selection a lookup is used to find the cost of the item this is shown in a calculated field unitcost.

The line entrys are stored in a portal called purchaseorderlineitems.

I need to apply a discount to this unitcost (ie Linetotal = (unitcost-discount) * qty, this is fairly straightforward.

But, I need to be able to apply discount as either money (ie

What you are trying to achieve can be accomplished by using the auto-enter by calculation option.

However to get around the circular reference message, you will have to enclose the reference to the other field (in at least one of the formulae) within quotes in the Get Field(" ") function.

Then whichever you enter, the other will be calculated - but only if the field is blank (previous values will not be overwritten).