Mine has 4 time settings for weekday and 4 for weekend.
I wouldn't go more than 2-3 degrees on the set back, it takes quite a while to come up to temp (1-2 hours), then overshoot and long time to cool down like you said. You are not saving that much coal, then just leaving it at a certain temp.
It will maintain more constant temp if you don't have to feed it alot, then to make up the 5-8 degree difference, it will keep feeding until the temp. is satisfied and then it gets ripping.
I have mine pretty much set to 65-67 all the time. I just tap it up or down, then it will go back into auto mode at the certain time.
My Feed rate is 15-20 most of the time and maintains 67-68. I bump it up to 70 in the evening when we get home, and doesn't take that long to come up.
Do a search in this section for SETBACkThermostat overshoot