My current tank is 18 high and my next tank is 18 high. its very manageable. 18 borders on a "shallow" tank and 20" is closer to show size, really up to you. By clearance do you mean between the ceiling and top of tank? What light are you going to run? Have you seen the thread in the SPS forum on RC: Poor mans Elos tank? Very similar and from GC.
Edit heres the link: http://reefcentral.com/forums/showthread.php?s=&threadid=1382521
Edit heres the link: http://reefcentral.com/forums/showthread.php?s=&threadid=1382521