Since PT cannot use the Grand Total to calculate an item,
add two fields that calculate the totals and
add yet another field to calculate the percentage.
The result might look like this:

Sum of Value Date
Area 1/1/07 2/1/07 SumDat
A 100 200 300
B 500 600 1100
SumAB 600 800 1400
PctAB 43% 57% 100%

In your source data, add dummy data for all
the months of the year.
SumAB has this formula:
=A+B
SumDat has this formula:
='1/1/07'+'2/1/07'+'3/1/07'+'4/1/07'+.....+'12/1/07'
If the initial source data does not show all the dates,
it will not allow you to enter this formula.
PctAB has this formula:
=SumAB/(SumAB SumDat)
formatted as Percentage.
In Table Options, uncheck Grand Totals.