For those that might be interested...
I put together an example file on the pure math behind calculating course intercepts. It's a workbook in Excel that has two example sheets that lay all the formulas bare with corresponding pictures from MoBo to help with visualization.
If the math interests you or for some reason you want to build your own intercept calculator with Excel rather than plotting the intercept with MoBo you might find it helpful.
Intercept Math in Excel
Incidentally... this is how I started out with determining intercepts and I hated it! That's why I built MoBo.