Minutes between creation of a lead and the first call

Minutes between creation of a lead and the first call

Hello. I want to measure the (business) minutes between the creation of a lead and the first call. I have two seperate modules: Leads and Calls. When I combine them into one pivot table, I am not able to substract from each other. So, I created a query table (Oproepen per lead). In this table, I am not able to make a formula column, so I created a statistic formula. After that, I created a pivot table (see image). However, it shows all the calls with each specific lead. How can I get only the first call in this table?
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