Staggered sum product formula

Hey all,

I'm working on a staggered time cash flow project. I'm trying to figure out a better way of calculating the formula in the yellow cells of the attached. The current formula works, but it is extremely manual and not very flexible. I'm 100% positive there is a better way of doing it, but have hit a mental block.

Any ideas?

The request should be pretty clear in the attached. ie find a better, more efficient formula for the yellow cells. I am not married to the current layout, and would actually like to put some things in Excel tables in case I want to do pivots or charts later.

Any help would be greatly appreciated.

Cheers.

Place the following formula in B6 =SUMPRODUCT($B6:B6,N(OFFSET($B17:B17,0,COLUMN(B17)-COLUMN($B17:B17))))
Place the following formula in B7 =SUMPRODUCT($B7:B7,N(OFFSET($B18:B18,0,COLUMN(B18)-COLUMN($B18:B18))))*-1
copy B6:B7 and paste it on the other columns