I haven't read the whole thread, so not sure if someone has already pointed this out. But when heating milk using a water jacket as the method of heating, turn off your heat source when the milk is 2-3 degrees lower than the temp that you are wanting. The water jacket is hot enough to bring the milk to the correct setting temp without any further heating needed.
I do agree though that most culture, either thermo or meso, have a fairly wide temp range before you start to render them either inactive or useless, so a degree to two here and there, will, for the most part, not make much of a difference to the final cheese.