Er ... think about it. The 30' is taught as being about right at and close to sea level as an engineering approximation - there's no particular reason to suppose that it's linear all the way up to vacuum, and plenty of reasons to suppose it isn't.
Here's one: if it's 30' per mb all the way up, and it's 1000mb at sea level, then you'll encounter vacuum at 30*1000 = 30,000 feet. Most of us have been for rides in airliners which fly at higher than 30,000 and still have sufficient air for the wings and engines.
(You could try working out the next level of approximation using A level physics, but I suspect you'd fail as you wouldn't be able to work out the temperature gradient without additional information. Personally I'm not going to try.)