Bookworm,
I have recently done a limited amount of research on the 30ft or not height difference per mb and I can confirm that WBS is correct. The 30ft per millibar difference is only valid up to (very approx) 20 - 23,000ish ft. At 35,000ft the difference can easily be 70ft per millibar! Ring your local Met man for confirmation.
Regards, ADIS