To calculate your prior week amount you could use the following:
IF DATEDIFF('week', [Parameter], [Date Field]) = -1
From there you could just use your calculation for Current Week to create your ratio:
SUM([Current Week]) / SUM([Previous Week])
Let me know if this doesn't work. I couldn't use your workbook because I have an older version of Tableau.
If this post assists in resolving the question, please mark it helpful or as the 'correct answer' if it resolves the question. This will help other users find the same answer/resolution. Thank you.
Thanks, that did it!