How do I create a formula that gives me a ratio of Lead Conversions per Week over Calls made per week?

How do I create a formula that gives me a ratio of Lead Conversions per Week over Calls made per week?

So using the two system-generated tables from CRM, Leads and Calls, I want to create a custom field for a report that will show me Leads converted by week, divided by the calls made per week. 

In the Leads table I have the field "Date Converted", and the Calls table has the default "Call Start Time" field.  What I would want of course is to group the data from these two tables under one date field, so I could have "Week Created" as the X axis on the report.  And then I would create a custom aggregate formula to calculate Leads Converted over Total Calls Made.  But, you can't create a Lookup between these two columns in the tables.  

Would this require a Query table (perhaps using Coalesce to join date column then Union All) or is there a more straightforward solution I'm not seeing?