Most of my Brent wheels are raised 4" (4x4) and most of the Shimpo wheels are raised 1.5" (2x4). This has been what is comfortable for the majority of my students.
What is comfortable for me: sitting at the wheel (with my left foot on a 3" height from two 2x4s) and my left elbow on my thigh I should be able to put my hand on the wheel head and have my forearm just a little above the whee head. If the forearm is hitting, it will give me rug burn. If I am too high up my wrist will hurt.