Yeah, you don't want to get below 75.

I know there are plenty of people who don't use belly heat, but I've used it during the cold months and will continue to. It can't hurt.

I put my probe on the glass, just above the substrate. Attached with a hot glue gun.
But ice placed them in different spots, you just need to check surface temps with a temp gun and use trial and error to dial it in.