You can achieve your Planting Index calculation in Tableau with this formula:
WINDOW_SUM(SUM([GDD]), -6, 0)
If you create a parameter with the values 7, 10, and 14 for the end user to manipulate, you'd want the calc to look like this:
WINDOW_SUM(SUM([GDD]), (([Days Parameter]*-1)+1), 0)
(This is because you're counting the '0' day, the date itself, as the 7th day, so technically you only need to look back 6 rows farther.)
If you plugged this Days Parameter into your Proposed Planting Date calc, you would only need one Excel sheet and one Tableau sheet, and then your end user can check out all three options in a single view.
If this helps resolve your issue, please mark my answer as the correct one. Thanks!
That works, but these leaves me with another issue. The reference line I have setup from the Date at the bottom axis, I would need this to move as well. It is currently setup to find the date when the Planting Index >= GDD Threshold and then moves back X number of days. When I build the new calculated field, when I attempt to use that field in the calculation, my line disappears. Thoughts?
Because the new calc is a table calculation, it converts your "Proposed Planting Date" field to a table calculation as well, so you'll need to specify how it should calculate it. Right-click on the pill on the detail shelf and select "Compute Using > F1." This will bring it back.