ExcelBanter - Trouble Forecasting

Not only trouble with Excel, but with Maths I fear.

I am currently tracking the number of hours worked on Field Advisors. And
these are currently being stored in a [h].mm format. While this is important
the real importance comes from the average over a 17 week period, this
information is in row 3, and works on the last 17 weeks (so in V3, the
formula is AVERAGE(F2:V2) )

Also set up, is an =FORECAST() that looks at these overall averages to
predict what the advisors average will be in 1 month, 3 months and 6 months.
Here is where I start to have trouble, since FORECAST takes an average of the
values, does this mean i'm taking an average of an average, which isn't
correct?

What I want though is some way of entering a number of values, these being
"Hours per week", "Date" and "Target Average" and getting returned any I
haven't entered. The Target Average will more than likely be fixed. So if I
enter say 01/09/06 the "Hours per week" might say -2, indicating that that
advisor should 2 less hours a week to reach the Target Average. Or I could
enter -3 in the Hours per week, the date might say 19/07/06, telling me the
target would be reached by then.

I have a feeling there is some function in Excel to do this for me, but my
Maths is limited, so I don't understand half the functions in the list, even
looking at the help file.

Cheers for any help!