Yes, it is a bit complicated.
Some advocate the use of hyphen in well-understood, others not.
I think if some other word comes after it "an well-understood concept'' I would say that it´s proper use (or it´s making into the dictionaries" you can see. Note, however, that adverb-adjective combinations are not normally hyphenated. For example, a “well understood concept” is not hyphenated (“well” is an adverb modifying the adjective “understood”).http://www.writingforward.com/grammar/p ... difference
As for with "well respected", it´s not proper use, so don´t use it.http://webcache.googleusercontent.com/s ... br&strip=1http://en.wikipedia.org/wiki/Hyphen
A "rule of the thumb" to know which ones you can use, is to get to the Cambridge Dictionary and see under the entry "well-"
it will display the "well-" fixed expressions like "well-adjusted", "well-advised", etc.