calculation field in a Pivot Table
 

I can't seem to be able to create a calculation field in a Pivot table. I
have a Pivot table that tells me the number of times a vendor has delivered
"on time". the original table has columns that state whether each line of a
Purchase order was delivered either 'on time', 'under' or 'over'. we also
keep track of any delivery issues that may have occurred on the same
sreadsheet. we call them 'NCM' and 'SCAR'. What I want the pivot table to do
is to take the total number of times a vendor delivered on time and deduct
from that the number of times they delivered over, under and ding them if
they have any NCM's or SCAR's; take that amount and come up with some type of
grading or score for that particular vendor. Can anybody out there help?