Churn rate from multiple months

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I want to calculate the churn on a period of 7 month for a strategy. The strategy starts on 20-March so we dont have data before this point. I can calculate the monthly churn but I cant understand how can I move from this point to calculate the whole churn for the period March-September.

Month       Lost+Down   Previous    Churn
Mar             -           -         -
Apr             205        235     87.23%
May              40        907      4.41%
Jun             370       1542     23.99%
Jul              90       1730      5.20%
Aug             700       2948     23.74%
Sep             850       2010     42.29%