I'm glad you got it working. It can be a really cool feeling to get these conversions working and discover all the sound information that's seemingly "hidden" in a stereo mix.
As for the formulas, there are no exact rules, but many stereo to surround conversions are based on some sort of manipulation of the sum and difference signals in the stereo signal. In V.I and its related plugins, the formulas are a bit more complex: over 300 connections among software modules, a dozen virtual speaker positions superimposed over an ITU 5.1 layout, bandpass filtering plus phase inversion for Movie Mode, 60Hz lowpass filtering for the LFE channel, and some other stuff.
If you want some references, check the "General Reading and Information" part of the
Additional resources section of the guide list. There are some really good resources there. Certainly the Sursound mailing list is a wealth of information.
Enjoy!
Steve.