Actually, I don't worry too much about 'odds'. The game works far more subtle then that. Best thing to do is to follow military principles. Bomb the hex with air. Hit it with artillery. Attack, but try not to exceed the stacking value. The value is usually 100, but goes up by 50 for every hexside after 2 you attack over. So if you attack from 6 hex sides (totally surrounded) you can use 300 stacking points. If you have over a 100, there will be a penalty, which basically means you will lose more and fight less.
There are also combat modifiers..shown as a percent..you get lots for different hexsides...but you can also get them through the use of cards. Try and maximize your attack by getting somewhere around 100 stacking points in a battle...try and have those units spread out as much as possible around the target...try and have those units 'pumped' with combat modifiers. Do bomb/bombard the hex before hand as it will kill a bit, and suppress more (that will not fight you during the battle). Try and keep divisions together (as they get the max divisional modifier).
“My logisticians are a humorless lot … they know if my campaign fails, they are the first ones I will slay.” – Alexander the Great