Excel Pivot table: Calculated field based on only the first row of a group

Question

I've got the following data and pivot table:

The Total column in the pivot table is the sum of the following calculated field:

=start-TIME(7, 30, 0)

I know that this calculation is wrong for what I want to achieve. I need to know how much delay I have had on each day to start the work. e.g. on 1-Oct-12, assuming I should have started my work at 7:30, 8:00 - 7:30 which yields 30 minutes delay, 1 hour delay for 2-Oct-12 and 50 minutes for 3-Oct-12.
So my question is, how can I have a calculated field based on only the first row of each group in a pivot table?

Answer 1

Instead of Sum in PivotTable use Min and then add a column to deduct 7.5/24.