Only things effecting score are those listed at the bottom of the screen there. So Your 'days to complete', level, Quests done, Difficulty, and battles lost.
18 Is definitely a good days to complete, but some have done it in as few as 6 or 7 days. So if you are looking for the BEST score, you are still off a bit
.
Oh, and difficulty of Normal vs Impossible will hurt your score a lot.
Good job though!