Calculate the total average days

Hello community!

I am stumped. So I am trying to do a "conversion" rate based on a start date from one table and a start date from another table. These two tables do not have a direct relationship, however, they are both connected to a main table through an ID. The main table includes the slicer # and the main table ID #. I am using a DAX where I pull the minimum date value based on the start date of each table, then I am using a datediff between those two minimum values.

What I need is for the sum of those conversion days for all of the main table #s and then I will divide that number by the count of main table IDs. 

Here's an example. Instead of it showing -254 I need it to total eveything in that table in order for me to divide it by the count. Hopefully this wasn't too confusing, Thanks!