You were very close to the solution. The order of the fields in your compute using dialog was slightly off. I have attached a version with the correct answer. The order of the fields in the compute using is very important as this tells Tableau how to run the calculation. In general, the leftmost fields on rows will be the topmost in the compute using. You can then add subsequent relevant fields to the compute using following the same sort of structure.
I have placed the calculation on the level of detail shelf so that you can always see the information when you hover over a mark.
Hope this helps,
Mocked SFL Rules Model.zip 36.5 KB
Thanks so much Dan! I didn't realize the ORDER PLACEMENT of the fields in the advanced dialog box makes a BIG difference.