FotRSU (and TMO probably) has a button on the menu for "Max Range at Current Speed" when you are on the NavMap menu (that is the Control Room view, of course, but the "Compass" button was pushed, and then the Max Range button highlighted):
Bubblehead1980 probably has that in his version also. While not 100% reliable, sometimes selling you short, and sometimes not close at all, it does give you an idea of when to expect "bingo fuel" for you to start back home. If you run TMO / RSRDC and put Traveler's Mod on top of that, you had better pay close attention to that function (Bleiente's Mod Pack also), since he made it even tougher. If you go slow, say "one motor speed" or Ahead One Third, you waste fuel, as well as time. If you go Ahead Full, you waste fuel. You might have to click on the hamburger menu below the throttle telegraph to bring up the Knot Meter, and from that, you can set a speed of 9 knots (or whatever), let the boat settle in on that, and see what that button shows again. The idea being, as Threadfin says, to stay on station as long as possible, to sink as many ships as possible within your assigned area. This is not as important in FotRSU as it stands now, since FotRSU is geared more toward "easy" initially. Plans call for an optional add-in to make the "boat ride" more "realistic" sometime in the future, but that will be a while from now...