custom calculation in calculating Turnover

Summary

need help to put an custom calculation in Turnover calculation

Content

Hi Everyone,

We have a requirement of customizing the delivered Turnover Dashboard with a custom turnover calculation.

When we select the turnover for multiple months or for years, the turnover should be calculated on an average of every month's turnover value as of last day of every month. 

Could anyone help me with some idea on how to achieve this requirement?

Thank you