It depends, but I usually (IXB/C) go ahead slow, 1/3 at most cruising or submerged. On long patrols, it is highly recommended. Also, on ahead slow one can survive the daylight submerged, wich is most helpful.
I save my fuel for emergencies and attacks
(As for the ruler, should you by any chance be using GWX, the GWX mod includes some sheets that should help, look for them in the upper left corner at the periscope station)