I've seen several instances where doubling the antenna hight typically adds about 20 percent more coverage over normal terrain. Of course this has it's limits since the earth hasn't been flat for many hundreds of years. I once saw a SMR operator spend about 100K to go from 200 feet to 450 feet with his system. He stated he was expecting to double his coverage. In reality it only added about 5 miles to his previous 20 mile coverage. He sure was disappointed.
Dex
