Using Calculated Items to calculate percentage of total

Dear all,

I want to create a summary table using pivots to check if items are on time or not on time. Items are delivered on a certain date and a separate column indicates that it was on time or not on time. My pivot groups by quarter and month in rows which is what I want and summarizes the columns how many are on time, not on time and the sum of both.
What I want to do is to calculate percentage on time (On Time/Grand Total) for charting purposes. I'm experienced with Excel and pivot tables, but calculated fields are new to me. This kind of calculation seams very common, but I'm lost for a (simple) solution. Is this even possible, with or without calculated fields?
I have attached an example with randomized data.

Any help would be greatly appreciated.

Kr.