I get 20 and 3/4 inches from desk surface to the top of the computer. One factor you may need to take into account is distance from the screen. At this size, I have it farther back than I had my 24" iMac.
Wow. I have my current 22" monitor 38" away from me. My iMac won't be able to sit any further back. I guess I will need to grow longer arms.