WilliamShakesBeer:
Two things:
#1: like stated above, the temp as measured from the outside of the carboy/bucket is always going to be 3-5 degrees cooler than it actually is in the solution. The yeast produce a bit of heat energy during active fermentation that accounts for the difference.
#2: I aim for the lower end of the optimum range. This leads to a SLOWER, but CLEANER fermentation, which is the point of controlling ferm temps in the first place. If you are used to 2-3 day fermentations, you are going to have to be patient with temp controlled ferms. I usually give them 7-10 days under temp control, then another 10 days at room temp in primary before I bottle or keg.
So, if it were me, and the optimum temp range were 68-72, I'd probably put my temp control on 68 with a 0 offset, so it would hold pretty darn close to 68. For a typical ale yeast, where the opt. temp. range is like 59-70, I set my temp control at about 63 with a 3 degree offset, which means the ambiant temps would stick around 60-63, and probably be about 63-66 in solution.