Main thread dealing with variables and HoI mechanics:
http://www.europa-universalis.com/forum/showthread.php?t=101944
Mithel's Starfire Historical Mod, mod focused on the realistics economical and combat simulation:
http://www.europa-universalis.com/forum/showthread.php?t=123457
Since it was quite old thing, I couldn't find exact link to the series of posts about that, but if you ask about that on any of those threads I'm sure you will get the answer. And reading them is very useful for understanding HoI mechanics in general.
BTW, I still got a lot of db mismatch messages during the scenario loading - is it normal? I'm 100% I've 1.05c and everything installed right.
And one more thing (you probably already know about that) - German leaders list got wrong name, so mod uses WW II leaders instead of it.
Saying all that, I have to add that I had great time playing it, even with all those bugs. Great work!
EDIT:
One more link to the thread about Ground Defense and ground defense efficiency modifier:
http://www.europa-universalis.com/forum/showthread.php?t=136035