About the data of Site Usage Analytics

Dear all,

I am new to SharePoint and I found that there is a Site Usage report. I want to download the excel file "90 day site usage data" to re-build the line chart on my side.

Do anyone know what is the formula for calculating the "Avg time spent per user"? More precisely, how to get the "Avg time spent per user" by using the excel file downloaded from SharePoint.