With mass combat, fist try reading over the rule pdf a few times to get a proper feel for it, a mock battle out of game will help.
As for counter hit points you need the number of men/counter, counters/unit and the soldiers con mod, plus any feats that affect HP (like toughness, that acts as +1 con mod). Its all there in the pdf how to work it out.
I mostly hand out around 2-4k xp as a base number to the party after a good session, and they are at levels 7-9, depends how quickly you want to deal with higher level characters.