Calculate End of Month Balance

I have 20 vendors. Each vendor has a customer base. I want to calculate the customer base at the end of the month in Power Pivot / DAX. The calculation is:

Beginning of this month customer count (same last month end of month customer count)
Plus New Customers
Minus Dropped Customers
Equals this month end of month customer count

I think the way to do it is to have a calculated column in the Power Pivot table called EOM Count. The formula in this column would be:

EOM Count from the preceding row plus adds from current row minus drops from current row.

I haven't worked out the formula, I want to be sure I have the concept correct first.

Also, since I have multiple vendors, each with their own ID, I should be able to add the EOM Count field in my pivot table, then filter it in the pivot table when I add a vendor column. Is this correct?

Thank you,
Bruce