Yeah, I would kill for my horse to get scores in the 20s. Lower is definitely better. They give you a dressage score as a "penalty" score, then any additional penalties you acquire on stadium or xc are added to that penalty score. On his record, it should have his penalty points for each area and then the total number of penalty points he has.
Beginner novice is a max height of 2'7 (or up to 3ft w/ brush), novice is a max height of 2'11 (or up to 3'7....i believe...with brush)
Hope this is somewhat helpful, I'm pretty new to eventing myself.