CalculatedItem suppress running total

I have a pivot table like this:

rows = date
columns = costset (ACWP, BCWP, BCWS, etc)
data = value

I've created calculated times for CPI and SPI
CPI = BCWP/ACWP, SPI = BCWP/BCWS

I want the cpi/spi for each period to be calculated off the cumulative-
to-date values, so for the value field I chose summarize by Sum and
Show data as Running Total in date.. This sums costsets up correctly
but then it also sums the calculateditems which I don't want

date ACWP BCWP BCWS CPI SPI
Oct-06 99.42 100.47 100.47 1.01 1.00
Nov-06 150.6 180.83 402.53 2.58 1.27
Dec-06 242.57 283.39 726.79 3.70 1.58

Look at Nov-06, it's cpi is 2.58. That's the previous period cpi plus
the current period cpi. If I turn off the Running Total, I just get
the current period cpi (which is accurate at least, but not what I
want)

date ACWP BCWP BCWS CPI SPI
Oct-06 99.42 100.47 100.47 1.01 1.00
Nov-06 51.18 80.36 302.06 1.57 0.27
Dec-06 91.97 102.56 324.26 1.12 0.32

Is there a way to enable running total for the value field but not for
the calculated item?

Thanks
Woody