Can I add a row showing percentages to pivot table columns?

I have a simple reporting template for outstanding debtors that uses a pivot table to calculate each debtor's balance whenever I update the Invoices tab. The pivot table is grouped by date as Months & Years. Attached is a sample workbook showing what is not working (yellow highlight) and how I would like it work (blue highlight).

My boss wants to see a percentage that each month represents against the grand total. I know how to add a calculated field by row, but cannot seem to do it by column.

I have worked around the issue by adding formulas in row 4 but each month when I update the data and refresh the pivot table some of the percentage formulas report #REF! and I have to manually set them up again.

Could someone advise me whether this can be set up so that the percentages will automatically update please?

Thanks for taking the time!!!

As far as I know, you can only add columns, not rows, for calcs

stasinek Sorry for off-topic interjection:

Although there is no official rule regarding this behavior, we request that wherever possible both the question AND the answer be provided in substantive detail here within the thread. An attached workbook is an excellent aid for posing a question and offering a solution, but solely doing that with no in thread explanation makes it difficult for researchers to understand or consider the Q & A of this thread without downloading what may be a pointless doc to them, if they can do that at all. Doing that also hides the content from search engines so others may never benefit from this.

I'm sure you understand, and we look forward to seeing you post your formulas/macros in your posts for the searching benefit of all.

Thanks again for all your hard work here!

Alternatively, you can add a column on the invoices sheet to calculate the % per business unit per month ( I calculated only % for open amounts > 0).

Many thanks for all of your suggestions folks. I will not be able to look at them for a few days but rest assured that I greatly appreciate your help.