I need to do a screen replacement on a pad where I can only see two of the corners. The planar tracker isn't working very well. Is it possible to track only one of the corner pin verticies? Then that one at least sits where it should be, and then I can place the last three manually? And is it possible to lock the length of the line between the verticies of the corner pins? So that if I move around one of them, the distance to the next will stay the same?