V 9.2

I have data for work tickets open and closed.

I need a line chart to show the Monthly Closure Rate.  Sounds easy...

However some of the calculation are based on open data and some on closed date and I don't know how to combine the two.

I sadly inherited this excel spreadsheet from the person that wants the chart.  I know there must be an easier way than what is in excel, but I cannot translate it to Tableau.

I have attached the excel sheet and the tableau data.

any help is appreciated and needed!!