Still, doesn't RFB has a tool for estimating maximum range in present speed, as in TMO? It is indeed the best tool to avoid running dry. Constantly plot returning courses and see if you can make it, if you have a small margin, an ample margin...
A good idea, if you think you are patrolling on the fringe of your range, is to adopt the following procedure: once every few days, maybe even a week, plot a course back home and check how much you can do. But if something new happens, such as finding a convoy and shadowing it, do that check constantly as you burn fuel at an alarming rate when on top speeds.
If you are far away - say, in metropolitan japan when your base is in pearl - a couple of days' chase can easily turn a confortable return margin into a dry-out.
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