To adjust Position value, Premiere Pro was designed to work on this in the Program Monitor.Just click on the 'position' property in the Effect Controls panel, this will show you the position on the motion path in the program monitor, so adjust the value by moving the position point (not the anchor point). If your playhead is right on the position keyframe you wanna change value to, moving the position point in the program monitor will set it to its new value. If you are before or after the position keyframes you created andyou move the position point in the program monitor, it will create a new position keyframe value.
This is really easier than adjusting position value in the graph editor, so that was left to 'scale' and the otherproperties, since those are hard to adjust in the program monitor. Remember, Premiere Pro is for video editors,so the simpler, the better... But if you feel that this could be a useful feature, you can open a feature request.