OK, I'm getting better at this, but I still have some problems that revolve around how closely to reach and maintain a desired temperature. Once a given temperature is reached, it seems pretty easy to maintain it because the mass of the water/whey/curd/pot metal holds the calories pretty well. Since I've been using a double boiler, I've noticed that if I take the pot out of the water before the target temperature is reached, the thermometer continues to rise--often past the target temperature. When the temperature appears to drop, I can put the pot back into the water for a short time and no matter what I do, the temperature appears to overshoot the target by a degree or two. With practice, I've managed to get the variations to within about two, maybe three, degrees. I assume this is because the metal of the pot has absorbed caloric energy from the water that is at a higher temperature and continues to transfer heat into the curds even after being taken out of the water bath.
My question is, how critical is hitting and keeping the EXACT temperature? Is there room for tolerance? Is it more critical for different cheeses? Can any mortal actually hit and keep an exact temperature using metal pots with water on a stove?