Calculated Field within Pivot table

Sir/Madam,
I have been racking my brains to try solve this issue.
Basically I have a table of data which I need to perform a calculation on. It is a summary calculation for the day based on 4 or 5 different entries that day.

The attached picture is an excerpt from my table. For a particular machine (named Carrig) 4 orders were processed. I need to determine the overall performance of that machine for the full day using a calculation called "Overall Equipment Effectiveness - OEE". This calculation simply divides the total pieces manufactured by the "available time*max design speed". Therefore for the example shown this should be [14280/(38*930)] = 40.4%.

In the calculated field, the formula sums the #pieces manufactured and the available time, which is correct. However, I want it to average the max design speed but it seems to sum them also, giving a result which is a quarter of the correct answer (because there were 4 separate entries).
I have tried to use the following formula within the pivot as a calculate field but to no avail
='Total OEE Pieces' /('Available Time' *AVERAGE('Max Design Speed (PPM)' ))

Any assistance would be greatly appreciated as I am running out of hair to pull!!!

Regards,
jb
Table excerpt.jpg
OEE pivot calculation.jpg

Great stuff xlnitwit.

Surprised power pivot is needed.
However, you gave me an simple workaround which resolved the issue.
Thanks,
jb

PS: I like your profile name... reminds me of fawlty towers!!!!!