This graph shows components of surgical time during a typical surgical case:

red bar:   primary surgeon needs to be present and is present

top yellow bar:   surgeon needs to be present, but isn’t–he’s late and everyone is waiting for him

bottom yellow bar:   surgeon could leave and allow assistants (residents, PA, first assist) to finish

top orange bar:   this is when the patient is being positioned and prepped.

bottom orange bar:   this is the time it takes to close and dress the wound, and take the patient out of any unusual position or device

Analysis:

Most surgeons will be in the room for the total of the yellow and red bars.  The first yellow bar is purely wasted time by the surgeon.  The second yellow bar has trade offs in that by staying, the surgeon could be shortening the length of the bottom orange bar -the wrap up time (but this is by no means certain).

The top orange bar is when the surgeon is not necessary, but he can dramatically decrease this time if he is present to coordinate and clarify.

Depending on what you want as your constraint (usually, but not always, whatever is in shortage) you can have the surgeon assisting before and after the time he absolutely needs to be available for the case.  Again, the top yellow bar is an exception to trade offs as it is totally wasted time.

Cost accounting would never catch these important facts, nor clarify the tradeoffs involved.