Pivot Chart - unusual average conumdrum

Hi

I have a series of Spreadsheets for Speedway results. (Motor bikes)

On any particular sheet, a riders average is calculated as ((Points+Bonus)/Rides)X4 - this gives a consistent average for all team members based on a usual set of 4 rides per meeting.

The Chart to show this would have columns
Name/Rides/Points/Bonus/Average/wins/seconds/thirds/fourths/Exclusions

However, when I create a further spreadsheet, to collate multi seasons with a pivot chart, the column for Average doesn't work, as it doesn't fit the 'Summarize field by' criteria, IE it isn't a Sum of averages, or a count of averages, as it will always be as per the calculation above.
I would prefer to have the Average column in the same place, and the only way I have thought to do this is a regular formula outside the Pivot.
This produces several 'problems'
a) It isn't in the place I want it
b) I want to sort the table by Highest to Lowest on these averages, which obviously works automatically on the Pivot data if I tell it which to sort.
c) As seasons progress, and riders are added, I have several changes to make, not only in Source data, but then also adding the extra manual formulas.

I hope all this makes sense, and wondered if anyone can think of another way around this?

Cheers
Neil