In my large tubs, I dont even worry about it if the cold end drops below 70 degrees. I mean, how hot is the room the rack is in? I doubt its 55 degress or something really, really cold. The snake will gravitate to the area its comfortable in. As long as you have a warm side, I think you're fine.
Sometimes I have the cold side, much...much colder tha 70 degrees (in the winter anyway) and my cold side is regularly 68-75 in the summer.
I think people (not just you, but me and alot of others) get the impression that snakes will just curl up on the cold side and die, this hasnt been my experience at all. They know how to thermo-regulate on their own. I really believe people worry about this because the whole heat-rock situation where they will curl onto the heat-rock until they burn. However, I do not believe the snake would go up to a heat-rock that is blazing hot and curl around it. The mistake is when the rock is only warm, the snake curls up and then it slowly cooks. This is totally different from having the cold end at 68 degress. In my opinion.
~Alan