These days, 20% is attainable only by experts at the lower levels, and it is virtually unattainable if you go much higher than the 16s.
See the section on ROI in the SNG forum FAQ:
http://forumserver.twoplustwo.com/sh...amp;page=0#roi
Please note, you would need a large sample size of games to accurately predict your true ROI. As in, 500 to have any idea and 1000 to make an educated guess. So no need to obsess over your ROI until the numbers begin to normalize and you can read something into them.