How to Calculate Multiple Rows

I'm using the SP v3.0 Inventory template. I want to calculate multiple rows based on the contents in the Title column. This formula works for one row >>    =IF(Title="2 x 4 x 8",(Quantity*5.33))

 Title          Quantity     Board Footage
2 x 4 x 8     251           1337.83
2 x 4 x 10
2 x 6 x 8

How do I make it work for multiple rows?  And when the Title changes to 10 footers the multiplier changes from 5.33 to 6.66.  And it changes again when the size changes to 2 x 6.  Thank you.

John Schmidt