Use 'on the mark' when complementing someone's accuracy in predictions or assumptions. Your analysis of the market trend was on the mark.
Pro Tip 2/3
Not Literal
'On the mark' often refers to non-physical accuracy, like ideas or statements, not hitting a physical target. Your interpretation of the poem was on the mark.
Pro Tip 3/3
Subjective Perception
Sometimes 'on the mark' reflects a personal opinion on how correct something feels, not just facts. I think your opinion on the issue is on the mark.