Calculating "Weighted Average" but excluding certain items

=(SUMIF($G8:$G35,"f",$R8:$R35))/(SUMIF($G8:$G35,"f",$I8:$I35))

The above formula works fine. However, the R cell I am referring to is a reference to another cell that really is another calculation (we will call the calculation in that cell "I x M".) Is there anyway to get this whole calculation in the above formula within the need for column R? What I mean is make the "$R8:$R35 " in the above formula something like "(SUMPRODUCT(I8:I39,M8:M39) "? When I try to do that I get an error which logically I understand. However I cannot figure out how I can do this without a whole new column.

My goal is calculating a weighted average function in one formula using only three columns: "G" which is the SUMIF condition field (I only want to calculate items that say F in that field) , and "I" and "M" which are what I need to calculate the weighted average.

Thanks,

Works great!!! Thank you, I didn't know the SUMPRODUCT command had that kind of flexibility but I knew there was a simpler way of calculating this.