I don't use the 4th rule. When I get to my patrol area, I take a quick note of how much fuel it took to get me there, so I start heading back to base when I've got a bit more than that much fuel left (I usually use around 10% as the 'fudge factor'). So if I get to my patrol area with 80% of the fuel I started with, I turn for home when I'm down to 30%.
Obviously this changes if your final patrol area is a long ways from your initial patrol area, but I think it's a decent rule of thumb.
Another thing to consider is planned refuelling stops. For example, if you're based at Pearl and you're going to patrol in the East Indies, it might make sense to stop in Darwin (or Tulagi, once it's open) and top up your tanks before you go to the patrol area. You can also stop there on your way home. That can extend your time on station by quite a bit.
Of course, if you're patrolling around the Japanese home islands, there's really nowhere for you to go.
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