Pivot table calculations
 

I have a pivot table with the following layout with hours being the data:

Elapsed Hours
Plant
Machine
Downtime Reason
Plant
Machine
Downtime Reason


I want to add a calculated field for % of downtime by reason, but can only
get it based on the report total instead of the subtotal. Is it possible to
do a calculated field by subtotal? Also I want to add a calculated field as
a % of hours available. The hours available is a constant amount in each
record my data source. When i try the '% of Base Amount' formatting I keep
getting a N/A value. What am i doing wrong?

Thanks in advance.