Thanks a lot for your quick reply. The formula you gave works. Seeing as the answer is so obvious I started looking at why I had problems. I was trying to multiply by a decimal number, like 1.1, and I am not able to do it. Is there something to take into considerations with decimals? Here is the formula I am setting up:
<!-- $>=(($4 * 0.8)+($5 * 1.2)) -->