near the mark; pretty close; not a bad guess
Used when a statement, guess, or description is not exactly correct but is reasonably close to the truth. Often follows a conjecture or estimate.
His prediction was pretty close to the mark.
I think that explanation is not far off the mark.
A set phrase combining the negative form of 当たる (to hit) with とも (even if) and the negative of 遠い (far), literally 'even if not hitting, not far'. The exact origin is uncertain, but it has been used since at least the Edo period to express approximate correctness.