So to get the SumProduct, the formula is actually very similar to the one you've posted!
So from your attached the formula in Excel is
and for all rows (unfiltered) this is 56.8
In Tableau the formula is;
SUM([Amount in USD]*[Payment Days])/SUM([Amount in USD])
As you say you can then use a quick filter to just look at January, for a particular supplier (as per the attached).
Hope this is what you were after, but if not please post back
Sum Product SR.twbx 46.2 KB