Well, this thread, e.g., has 40 posts from only 12 different posters. That is a ratio of more than 3:1. Therefore, 28 posts originate from return visits, corresponding to a proportion of 70% returners.
Of course, this calculation may be not representative for the whole forum, but it is an estimate on an empirical basis, not a simple guess.
On the other hand, these calculations are not really important, because your second misconception is that you mix up members who abstain from voting with those who give a negative vote.