How does the surgeon know what the result will be if working on a patient laying down if marked when sitting up?

Good question.It is the combination of experience and it is functionally better, easier, and safer to perform the procedures with the patient laying in the supine position.  I know this is confusing to some patients but when you think about it there are many professions where something is made or completed when it is in a different position.  For example, when your clothes are tailored they do not do the stitching and material cutting with the dress or suit on you.  Or when a carpenter fabricates a wall it is commonly constructed on the ground before it raised to its vertical and final position.I hope this helps.  

The translation of preoperative evaluation into operative plan is the reason that facelift should be done by surgeons with vast experience. Achieving outcome comes from knowing how a manipulation in the operating room will translate into a postoperative result. You can't learn this from the weekend course.

Things look very different when upright as compared to when lying down, that is why surgeons do their markings when patients are upright.  With extensive experience, a good surgeon then translates those markings into a well executed surgery when the patient is lying down even though lying down the markings may seem off.