Pivot Table % based on existing Pivot Table columns

[bessegap]

I'm hoping someone can help me out, I have an existing pivot table with four columns

Standard late room
Surgeon
Last Case
Late last case

I want to include a column to calculate the % of last cases as a total of cases and average late last case time... any suggestions on how I can do that, I've tried a few hundred ideas over the last few days with no success

thanks