compare values with previous period

Hi! 

i have some list of customers by months https://dropmefiles.com/hqQii

And I stucked, trying to compare list of customers to previous month.

For example in october there are 4 customers (2,5,7,8) who came also in september, so in october "comeback index" will be 4/ 10 (amount of customers in september) = 25%

In november it will be 6 customers (2,5,7,12,13,14), who came in october, so it will be 6 / 8 (total customers in october) = 75%