I might suggest that 75 is ok but if you are seeing 75 it should be with a proven accurate thermometer and that it simply could not drop below this. I would recommend no cooler than 77 ever as the error rate of most cheap thermometers is at least +/- 2ºF and some (accurites I have been told 2ºC (4ºF) up or down that would make a reading of 75 actually 71 you see my point.) Not every one has accurate thermometers as they are cost prohibitive. I would check the specs of what you use and assume it is on the far end. (if it is 2ºF +/- 75 would really be 73 so 77º would be prudent. unless you had an accurite then assume 75 is 71 so 79º is as low as I would choose in that case.