Calculating whether new customers or Existing Customers

I couldn't share sample data here but my data is item level data for say each invoice I can have multiple entries because same invoice has multiple items so against each customer
I am creating column which will give me first purchase date of that customer then using that column I am checking if invoice date = first purchase date then customer is new else existing i did this because i want to create a filter of new customer and existing customer based on that i want to see which customers are new in other visual .
If i select some period then i am getting customer only related to that period based on customer status column this is working good until here
problem now is when i shown total count of customers in scorecard i want to show total customers if something sleeted then then base on that count of new or existing customers but say i have selected a month then in that month my total customers are 500 without customer status selection if I have selected new customers i get count 200 because new customers are 200 this is good but
when i select existing customers then i get say 400 this is because it contains existing customers this is possible that same client can be new or existing of selected month because that customer have purchased multiple time so same customer got counted in new customer also and existing customer also i want to show count of new when new customers is selected but when existing is selected then total-new so i can get correct count and my total customers will be = New+EXISTING