This thread has very complete measurements on power use during “normal power on”, s2idle, and deep:
The expansion cards have a non-negligible power use during s2idle/deep.
For your measuring accuracy: I wouldn’t go with “estimated time till drained”, since that involves a highly variable estimate of power use-while-on. I think you’ll get more reliable results by querying the estimated charge of the battery (in mAh or in W). It’s not perfect either, but the runtime estimate is based on it, so you’'ll get more stable results.