Multiple criteria if statement

I have a number of units, a rate per month these units are absorbed, and a cost per unit of absorption; not to mention a date function coinciding with them. I need to subtract the units at the given rate per month beginning with the specified first month. I need to use up no more than the total number of units.

At the moment, the cells under each date is as follows:

=IF(EOMONTH(H$1,0)>=EOMONTH($E2,0),$D2*$F2,0)