The actual ratio of altitude change to pressure change is closer to 27 feet per millibar.
ft, I can see you have a good understanding of this topic, but might I add a note of clarification for those that don't and may take away the wrong message from your accurate observation as far as this question is concerned?
The ratio of altitude change to pressure change increases as you go higher.
Whilst 27ft/mb is indeed a good figure at low level, i.e. sea level to 1,000ft, 30ft/mb is a better average in the range 1,000 to 5,000ft ... and by the time you're worrying about the impact between 25-30k ft, the figure has grown to >65ft/mb!