Calculate percentage based on values in multiple tables

[Reetz]

I am new to DAX.

I have 2 tables: Order Table and Hospital Table, that I am trying to calculate a usage rate. The problem is that I won't always have data for every month in one table that corresponds to the same month in the other table.

I only want to perform the calculation when I have corresponding data, by month, in both tables. 

I am also connected live to my dataset. Is that the same thing as Direct query?

I need to perform 2 calculations:
1. Usage rate by month for those months where we have matching data in both tables
2. Total average usage rate for those months where we have matching data in both tables.

The usage rate calculation is Order Table[order history]/ Hospital Table[PT days]