I assume you already have a fact table with at least three fields, Customer, Period, and your percentage, right?
So getting the calendar by Month Year should be easy, either by using the Period field or linking to a master calendar
For your service month, specific to a customer, you could find the service start (min Period or a service start date per customer from a separate table), and create a table in your model with fields Customer and ServiceStart.
It might also be good to in addition create a consecutive MonthID, a number that counts the months from a certain point in time (say, January 2000), so we are now in month with MonthID, hmmm, 14*12+3=171.
Then join the ServiceStart to your fact table and create a difference between ServiceStart and current MonthID for your ServiceMonth.
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