New User: Using a calculated measure between different tables

Hi Everyone,

I'm quite new to PowerBI so I'm still trying to get the hang of it. 

My question is: How we can do a measure between different tables? I can obtain the revenue  

What I'm trying to achieve:

• Graph of Average Transactions Per Customer by Month i.e. [2-4]/[1-2] for each Month (MMM) along X-axis
• Graph of Average Reveue Per Customer by Month i.e. [2-5]/[1-2] for each Month (MMM) along X-axis

My datasets available (I may be linking the wrong sets? - Hope this is descriptive enough to follow)

1. Customers:
   
   1. Month-Year (in MMM-YY)
   2. Customers (Total customers per month)
2. Transactions and Revenues
   1. Transaction Start Date/Time (in DD/MM/YY HH:MM format)
   2. Transaction End Date/Time (in DD/MM/YY HH:MM format)
   3. Transaction Duration
   4. Transaction Unit (all transactions set as 1)
   5. Total Revenue of the corresponding transation
3. Dynamic DAX Calendar (self created)
   • Date (DD/MM/YY format) - This is linked to 2-1 above
   • YearPeriod (YYYYMM format based on date)
   • Month (MMM format based on date) - This is linked to 1-1 above
   • MonthYear (MMM-YY format based on date)