How to create a pivot table for customer information starting from a base year

Hello. I have information about customers that started filing contracts in 2008. I have the data (thousands of rows) in a format that isn't friendly for pivot table analysis so I'm going to normalize it. However, I realized that since some of these customers fell off (didn't renew their contracts) that the data will not make much sense. For example, year 1 has 100% of the customers, year 2 will have less, year 3 even less. The averages that I compute won't make any since. How can I adjust the data so that when I look at it in the pivot table the averages and other data will make sense. Thanks. Sample provided.