I have been looking to change my stem length, but found it pretty confusing with all the different lengths and angles available. The idea is to change both the turning radius from the head as well as the reach from the seat. Figuring out where the bars will end up is simple geometry, but difficult to visualize. I put together an excel spreadsheet to calculate show a graphic comparing several different options.
The file is for others to use. I guess it is a bit odd for my first post not to be a question at all! I found it useful when I was trying to find the best deal on a stem that would work. For example I found a 70mm 6 degree for $30 and an 80mm 25 degree for $17. By adjusting the height with spacers the bars end up at the same place, but the 80mm stem will act about 6mm SHORTER than the 70mm (because the distance from the head axis is shorter).