help with excell

I wonder if anyone out there can help me with an Excel problem i am having....

i want Excel to calculate a commission based on the following information the these columns:

gross sales
less expenses
net profit
% of profit

commission

the first part is easy and i can handle that...its the calculation of the commission that is giving me problems. The calculation of commissions is as follows:

if the percentage of profit is 20% then that is the commission amount eg: 20% of the net profit

if the percentage of profit is less than 20% then the commission amount is whatever that figure is: eg 19.8% times the net profit

if the percentage of profit is more than 20% then the commission amount is 20% of the net profit. eg: percentage of profit is 21.3% then the commission is 20% of the net profit...

if there is a loss (net profit greater than the sales) then the commission should reflect a negative percentage and therefore be deducted from any commissions due

i think that covers all the possibilities

Thanks in advance for any help you can give with this

Bert Hayford