In the rules, 2 levels of terrain hides a mech behind a hill.
Most mini's are approx 2" tall or less.
Make your terrain 1" per level, that way level 2 will cover the miniature and make it look like the rules as presented in the books.
If you make a level 3/4" high then try to claim full cover when standing next to a level 2 hill, you make look a little silly with the upper torso and head clearly visible on the map.
cheers,
Rob