Agreed; if you've got two fleets essentially sitting in formation with one another, they can be considered adjacent in game terms. After all, ships in close range of one another only need be in a 10km radius bubble around your reference point.
Beyond that, it's just a case of applying a bit of a workaround. In an RPG rather than a wargame, the GM may be throwing multiple ships at the players but if they only have one ship themselves, that's fine. You just use their ship as the reference point to draw ranges from.
Equally, if you are protecting a convoy of helpless merchant ships (all within close range of each other) from a pirate, use the pirate as the point of reference for ranges - it'll shoot at you and at the merchants, but you won't be shooting at the merchants so what range band you are from them doesn't matter.
Once you reach the scale of two cruiser squadrons versus several destroyers, particularly if they're deploying fighters which are moving to attack one of the destroyers but might be shot at by a second, the whole thing starts to become a bit of a mess. you could try tracking it on a number line but relative ranges between multiple entities don't quite work - thrust applied isn't quite the same as distance.
For a big engagement, as noted, just assume the entire fleets to be sitting at one 'point' exchanging fire with a fleet at another 'point'.
