First, I think you might get a lot of hints from this article Top 15 LOD Expressions | Tableau Software .
Then back to your problem.
From your descriptions, I see that you have several requirements, well, let's begin with this.
ie. if 1 customer out of 100 orders in month 1 and so registers growth, I don't want to show this as 100% of customers growing
Instead, I want to show that 1% is growing, 99% are not yet active
Is the ratio in the attached sample worbook (version 10.1) what you expect?
Ratio Sample.twbx 1.1 MB