Really depends on your tank. I set my alarm at 2 degrees F below the set/desired temp. My heater is on an inkbird that will alert me for both high and low temps.
I personally put it around 3 degrees below my current temp. I run my tank at 78 and the alarm at 75 so the alarm doesn’t come on every time I do cleaning or maintenance.
I have my heater on an inkbird with an alarm for 2 degrees low or 77 degrees. I have that plugged into a neptune power bar that shuts it off at 3 degrees either side of 79. but since i dont want the wear and tear on the neptune, the inkbird does all of the on off switching.