From my observations - in a Bell 206, at a medium weight, the difference in pressure from rotors stopped to just lifting off the ground (which is where the pressure difference is most noticeable) is about 50' on the altimeter.
Isn't that easier to use as a measure - it's really insignificant as far as performance goes.
An interesting effect, but of little practical value.
And for whoever it was that copied the stuff from my book - I don't mind. The plugs are appreciated!
As far as the 'bubble' stuff goes - in a perfect no wind day, there may be a slight tendency to push the helicopter when the thrust vector has been tilted, but I would defy anyone to measure it.
I seem to remember there used to be a requirement to measure static stability in the hover - turned out to be impossible to do.