the height/clearance is determined by UL or whoever listed the appliance. Most microwaves that we install set a minimum of 30" from the top of the range to the TOP of the microwave (this leaves about 14" between the microwave and the range. When I'm remodeling a kitchen I find the perfect height is to allow 33" above the range which lines up the BOTTOM of the microwave with the bottom of the cabinets (or about 18" above the range)
