I have 3 ideas for this:
1.) (This kinda can kill 2 birds with 1 stone if your using minis) Use the range measurer from the X-Wing game so like andything that isn't touching but is within Range 1 is Short Range, anything in Range 2 is in Medium Range, anything in Range 3 is Long Range and finally anything NOT in those Ranges is Extreme Range. also you can use these minis for the Space Combat Encounters.
2.) Each square on the grid could represent a 5ft distance with this you could say up to 30ft is Short Range, up to 60ft is Medium Range, up to 90ft is Long Range and anthing beyong 90ft is Extreme Range.
3.) Use a Tape Measure and say up to 4 inches is Short Range, up to 8 inches is Medium Range, up to 12 inches is Long Range and 13 inches or more is Extreme Range.
With the exception of Idea 1 the distances (I.e. 30ft or 4 inches) etc I chose were hyperthetical for the purposes of answering this question.
AND Kallabecca has a very valid point the range band for each map is different so you may have to tweak above ideas a bit if used