Not sure if this is the right way but here's what I do. To scroll text to the middle and then stop it, I create my first effect and and adjust the effect duration so that it ends right in the middle of my matrix. Then I follow it with the same effect with motion set to none. It can take some of fiddling to get it right but it works.
To play a sequence only once, I adjust the scroll speed until it seems about right and then simply play around with the effect duration so that it ends after one cycle. That usually only takes a few seconds.