Averaging data based on a Date range in a visual

I am looking to create a report on mark up data.  To make this report each entry has a start date, an end date, and a markup percentage.  I want to create a visual that shows the average markup month my month and the average markup is computed by averaging all the markups that active for that month (that month is within the date range).  I am thinking to do this I may have to make another table and filter the data 

Are there any other ways I may be able to do this?

I have tried this approach.  Made a new table with all the start of month dates and end of month dates and then wrote an expression to make a new colum using to try to get the average using these two colums but I get an error.  Is there a better expression to use to do this?

Date tableExpression tried