Jim, if I ferment a beer at 65, it will start at an ambient of 67ish, and then after fermentation begins, I will slowly drop the ambient down to keep the beer at 65. After it is pretty much done I will raise the beer to 68-with an ambient of 70ish. My goal is to keep the beer at an actual temperature, not allow it to vary over time. In your example the beer would start at 58ish and then if it took off climb into the 60s somewhere. But you would be fermenting it over several degrees as opposed to 65.