Basically, you're trying to find the regresssion for the function:

sales = base_sale + K * log( i )

where K = multiple/log( logbase ). There's no way by regression alone to determine multiple or logbase separately.

The above regression is not strictly linear, but if you tranform i to x via

 x = log i 

(or

i = e^x

), then you get:

sales = base_sale + K * x

which is a straight linear regression once you apply the transform correctly to your interval variable. Thus, you simply have to do some stat summation over your data set, and you're all set; the exact equations for that should be in any numerical math text (I don't see any Perl modules that do regression easily, but the routine isn't hard for this).

-----------------------------------------------------
Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
"I can see my house from here!"
It's not what you know, but knowing how to find it if you don't know that's important