manipulating columns

Hi all.
I have a pivot table made up, but I need to do 5 or 6 steps for the
final presentation.
Basically I have different columns that are categories of Land Use
Zoning for Sacramento County. I have on the left side of my table the
name of various projects(The Falls, Kamilos Gates, etc...). I need to
be able to figure out the acreage of the Land Use Zones for each
project. I have the acreages made up for each project for each type of
Land Use.
For example, Kamilos gates has 2 land uses, RD1-2(Residential Density
1-2 units/acre), RD2-5(Residential Density 2-5 units/acre).
Now, these numbers have to be mulitplied by a preset number (made up
the by the County Planning dept.) For the RD1-2 column the mulitplier
is .66, to get a "Dwelling Unit Count".
So how do I make up a new column that will contain the product?
I know this is complex, but any ideas would be helpful.