My observations on the pressure altimeter are that (in a no-wind situation) as the collective is raised from flat pitch, the altimeter starts to decrease in reading. This indicates an increase in pressure sensed by the static ports. The altimeter reading continues to decrease as the collective is raised, reaching it's lowest reading just as the helicopter lifts off.
The altimeter then starts to return towards the 'correct' reading and when the helicopter is out of ground effect, the altimeter is once more reading correctly (using known external references for height, as a radar altimeter is not accurate enough for comparison).
So, when OGE, the pressure altimeter is pretty accurate.
The difference in height/pressure at the 'just lifting off point' is minimal and makes no difference to any performance data. It's of academic interest, aside from modeling ground effect as has been so spectacularly done in earlier posts.