If you want a nice smooth movement then you need to put another Key one frame after your first key. So you put in 1(0),30 and 60 then you need to copy each of those keys adn paste them 1 frame up at 2, 31 and 61 (or 59) if you don't want to add to add extra frames. The other way is to turn all frames to Linear and you won't have any overshoot. I have 4 or 5 bug reports in about Poser's Spline curve and how it fails and what should be done to fix it.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13