Vishti closes the rotation of seven movable karanas and takes positions 8, 15, 22, 29, 36, 43, 50 and 57 of the sixty.
Position p occupies tithi ceiling of p over two, in its first half when p is odd and its second when p is even. So Vishti holds the second half of tithi 4, the first of 8, the second of 11, the first of 15, the second of 18, the first of 22, the second of 25 and the first of 29.
The fourth of those is the one to note. Tithi 15 is Purnima, and Vishti occupies its first half, ending at the moment of opposition itself, after which Bava takes the second half. The karana with the strongest caution attached to it in the whole scheme therefore falls, in every single lunation without exception, on the first half of the full moon day. That is not a doctrine, it is a consequence of Vishti being seventh in a rotation of seven starting from position 2; but it explains why almanacs publish Bhadra timings so prominently around the full moon, and why the caution is met so often by anyone timing an observance to Purnima.
Position 57 is the other one worth marking: it is the last movable karana of the lunation, immediately before the three fixed karanas that carry the month into the new moon.
A karana averages 11 hours and 49 minutes and can run from about ten to about thirteen and a half.