On Fri, 17 Oct 2008 07:31:02 -0700, Brazell
wrote:

Unfortunately that equation can get a little hairy and can exceed some of the
limitations for formulae. At least any formula that I have seen.

It's hard for me to deal in these kinds of hypotheticals. But if you have a
data table, it's usually easy enough to find the appropriate values for which
to use the FORECAST function by using one or more of the lookup functions, or
by using the combination of INDEX and MATCH.
--ron