I think you system is overkill for tomatoes. a great system I'm sure, just tomatoes are easy enough to clone in a glass jar in a window. ==>>
As for temperature, anything around 70° should be fine so I think the 66°-68° should work. I would forget about the heating pad. you said 64° was the room temperature and 68° is as high as the water is getting. the ultrasonic will generate some level of heat, so it sounds like the pad is doing almost nothing. at the very worst the water temperature will not drop below the room temperature. give it a few days at 64°-68° and if things don't move to your liking, add a small aquarium heater to boost the water temperature.

Thanks Grizzman,
I have cloned tomatoes using the glass jar method, and you are right They are quite easy, I started this project to increase success rate and decrease cloning time, with the intent of experimenting with Woodier stemmed plants latter on. I should learn stay more focused on my primary goals. I placed a 1" thick piece of styrofoam under the pad. The temp is at a steadier 68-69f and the table is a lot happier. So I'm going to go with it for now but I plan on getting an aquarium heater for the colder season and for tackling the more difficult to clone plants.
Thanks again.