Ah sorry, I should've included that.
The program should, if the button is pressed (even momentarily) complete one arc, of 180 degrees (from whichever side it is one to the other). And then stop. The next press should move it the 180 back the other way.
Taking finger off the button mid-sweep, the servo should complete the arc, then stop. (and this is probably the primary way this would be used)