Since coal heat is so steady and even in a stoker using the Coal-Trol I have found that it is more efficient to run the same temp day and night. When I first got the unit last year I experimented with setback temps, the savings in coal were minimal and the time it took to reheat the house and the resulting overshoot each day all but negated what I had saved by the lower set point at night.
As WNY suggested I would say that 2 degrees (3 max) would be all you want to set back at night. As has been posted (if you have the right firmware in the Tstat) you can play with the HLF to speed up the recovery from setback, but the resulting overshoot will run the temp at least 3 degrees above set point.
Of course every house is different and you just have to play for a little bit to accomplish operation the way you want it. The Col-Trol PID is set up for the slow responding stable nature of coal and those guys have done a great job trying to match all circumstances. Paul gave all the basic considerations for making adjustments and you will be able to improve the situation by following his advice.
My advice is to let the Coal-Trol and stoker combination do what it does best, keep your house at a stable temp (minimize or eliminate any night set back). I think you will find overall that it gives you the best use of your coal and doesn't cost any more than large differences in night setback.
This opinion in no way reflects the opinion of the Forum owner, its members or manufacturer's rep's.