Multi function cells

Hello all,

I have recently started using Numbers for more than just basic data logging, and am working on a simple currency converter. I trade on various currency exchanges and would like to automatically include trade fees etc.

I would like to be able to enter an amount for example in USD and then see how much EUR I can get for that amount of USD including the trade fees/commission.

Example: Cell 1 enter USD amount -> Cell 2 should then display the max amount of EUR I get after deducting trade fees.

This is simple enough and I have figured that out.

Now I would like to be able to enter an amount in EUR in Cell 2 and then Cell 1 should display how much USD I would get if I traded the currency the other way.

The problem is however Cell 2 has a formula in it and when I enter the EUR amount it replaces the formula. I am sure this is doable in Numbers however I have no idea what to even search for in Help or good ole Google to get this functionality...

Please don't hesitate to ask for clarification if necessary!

Cheers and happy calculating,

Kai