Calculate/Display Percentage in a Pivot Table based on an alternate Total

i have used a few Pivot tables.
In some of them when i have grouped or organised the data, i would like to display the results (sum totals) as a %, but not as a % of the total values in the sheet,
but as a % percentage of a target set outside of the sheet.

is there a feature in Pivot tables to do this,

E.g In a sheet there are 10 values, in the pivot they total 200, the maximum total would be 300, Can i display the values as a % of 300. How ?

As usual, Thanking you in advance

Baj