On Wikipedia, there is a list of phonetic vowels and their average 1st and 2nd formants.
Throughout the web are many more of such charts with varying frequencies.
So obviously some amount of tolerance exists, but I don't know what it is, nor do I think I could really come to a strong conclusion using the data I have.
What is the tolerance on formants?
P.S. if you know of a better formant chart than the one on Wikipedia, please share.