I use 8 kts (VIIB) and use my batteries when full, mostly during the daytime. At night I cruise on the surface.
Click under the speed dial) for being able to specify speed in knots. This way you will not only have plenty of fuel, you will have enough to overhaul.. say a convoy
Especially then, flank speed all you can. You won't feel bad, because you will have saved for that now