Need help with an additional validation script for multiple (A+B) X C calculations

Ok, so my D1, D2, & D3 columns all have the same calculations, but only one of these columns will be used per row.  My overall form has 6 "Polygon" rows.  I would like to create the form where any row can be used for any of the D columns.   I tried to use the simplified field notation calculation, but obviously it duplicates the answer (I didn't bother to format the last 3 boxes yet).  I ran into a similar issue earlier (with division), and it was resolved with a validation script (which someone graciously provided to me).  I am too ignorant to extrapolate that solution into this problem though...someone please help!

Thanks in advance