Ah, nevermind. I figured it out. Always seems to work that way, doesn't it....
Try sum([Donors Needed])*7
This is definitely an issue of the Prospects needed acting on the row level (non-aggregated), and count ID working across rows.
It's difficult to diagnose these issues without seeing what your data looks like.
This article from the Information Lab may help you understand more: http://www.theinformationlab.co.uk/2012/03/27/tableau-for-excel-users-part-2-calculated-fields/