Rob, an (almost) foolproof way is to chart up the distance to your patrolling grid, and then start going. Ask your navigator how far it is, and then how long you can go at current speed. If the value he gives you is at least double of the range to the grid, then you will have enough fuel to return. If the range you can run is below the double of the distance to get there, lower speed, and ask again. Lower speed has a major impact on range.
(Of course you will spend some time submerged, running on battery power, but you get the the point)
Sorry for my english, if this is unclear.
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