Subtotal pivot table data to show % split
 

I have a pivot table based on an external source.

There are 2 column items, one called Types and the other is Data, which
shows policy count and value. The 'Types' are listed with labels prefixed
with either "Commercial" or "Personal".

In fields separate to the pivot table, I'd like to show the % split between
Commercial and Personal, possibly using a subtotal function so that when the
user selects criteria on the pivot table, the value in the % split cells will
update accordingly.

I hope I've explained what I want to do in a way that makes sense, and I
hope there might be an easy solution.

Thanks