rather than using circles, like for sensor range, you could just connect fleets in reinforcement range with lines. fleets that are close to reinforcement range but not quite there can have broken lines to show how far they're off. the lines can be colored for fleets in range to reinforce your own, hostile fleets in range to reinforce each other, and possibly non-hostile fleets in range of each other as well.
i don't have a fitting screenshot, so the ranges here are out of whack, but just to show what i mean:
i think the only thing this doesn't show is how far two connected fleets are from
not being able to reinforce each other.
connecting every fleet with every other fleet in range might be too much in busy areas, but it could be limited either just to your own fleet as well as the closest hostile one, or shown only on mouse-over.