2 events tied together by date filter

I have 2 type of data tables: assignments and completions. I want to perform the following formula.

(# assigned_completions + # unassigned_completions) / (# assignments + # unassigned_completions)

My elasticube has assignments and completions as different tables thank link to a dimension date table via the assigned date and completion date.

If I have the date filter set for August and someone was assigned in July but completed in August. How can I reconcile these numbers so that if it completed in the date range then the assignment gets counted?

Any help is appreciated.