Does anybody here love phonetics and phonology like me? We could speak about it in this thread. I just checked the wikipedia mandarin phonology page and found out that I'd forgotten mandarin doesn't have /h/, but /x/. Now i'll have to get used to it again :x
Just for you to know, sinologists use "/x/" because it looks neat next to /k/ and /kʰ/ as their fricative counterpart, in the way /f/ is to /p/ and /pʰ/.
In reality, the most common allophone for /x/ is [X], a voiceless uvular fricative with optional trilling in Beijing (I guess that's what you're learning). During the summer I volunteered at my school during its "international education summer program", basically working with East Asians who had come by for two months to practice their English. I can assure you from all the Beijing Mandarin I used to hear that they really do have [X] for /x/, often as a trill when they were talking emphatically.
and how should I know if I'm doing the compressed vowels correctly?
By looking at the pictures of this article:
http://en.wikipedia.org/wiki/Roundedness
