Doors and windows are not a deduction, but an Addition to wall painting.
They have to be cut in individualy. Rolling is quick, real quick, brush
work is slow. Painting just walls with no windows in a small room may
take a few hours, cuting in wall paint around doors and windows and base
and ceiling trim could add 4x the work And I am not talking trim paint,
just walls .
If a painter came to my house and started measuring, I would show him
the door as I would know he is an amateur. Measuring is needed on large
jobs, not rooms.