Predicting the future can be a very deep rabbit hole to go down. FileMaker forecasting tools are going to be a bit more do-it-yourself than with Excel, but not by much if you're forecasting with any sophistication. Skeleton Key did a blog post series on using FileMaker to build tools for basic predictions; there's some use of the ExecuteSQL function introduced in FileMaker 12, but the concepts can be applied using FileMaker 11 just as well.
I have been doing forcasting for a while now with data in FM db's (sales, inventory and attendance numbers), and have for the most part found it easier to dump data off into Excel or R (well worth looking into http://www.r-project.org if you aren't already familiar!) and then reimport coefficents periodically. This made a more sense for me as I tend to work on fixed scope projects more than ongoing data streams, and also allowed me to easily look at several models at the same time.
As Jbante said, FM doesn't give you much help in this realm so you will need to create all the formulas if you are looking for "longterm" built in forecasting. Depending on the amount of data and the complexity of your model, this could become very taxing on your overall database performance. It sounds as if you are still trying to determin how best to model your forecasts. This I would definietly do outside the scope of Filemaker. If you have historical data that will be great. Play with it in Excel or R and see what model gives you the best fit. It will then be easier to recreate the math in your model within Filemaker if that is what you decide to do.
One last thought... don't overlook a dumb and simple "at the current rate" calculation. Despite all it's limitations, you can get a usefull number with very little overhead.