My amateur one installed fairly well about 6' off the ground with vent shield and little sun and got to 74 or 75. HGR was over 70 as well. I would believe anything between 70-75. Rooftop is not realistic, I agree, but some sites say it is a good way to do it. On a day like this day in particular, I am almost always going to be higher than you without any valley protection like you have.