Help me please sum and formula field related

Hi,

please help me do this, there are similar threads but not exactly what am trying to do.

Am building a very simple purchase order system and the form needs to collect a few item fields, but the problem am having is when am trying to place a field with the result of multiplying item1.quantity*item1.unit.price how can I obtain a result of this operation in order to be able to later get the sum of (item1.quantity*item1.unit.price)+(item2.quantity*item2.unit.price). I can't use a formula field here because I will add another operation to multiply the result of item1+item2 to a certain Tax percentage and formula fields apparently don't use the result of another formula field. I hope I explained myself correct.

Thanks in advance.