Does anyone have any economic forecast formulas?

I don't need them to be accurate, just believable.

I'm specifically looking for population growth compared to gdp per capita. I'll accept anything though. Something that is developed from empirical data is preferred, but not necessary. Oh, and it needs to be public domain, free for use with modification. Like supply and demand. No, I'm not going to be spending money using them. I'm just a data hound.