If you're looking at the E6B, it's nothing other than "time remaining at this fuel consumption" times "current miles per hour" -- a REALLY simplified version of what you have.
Don't trust it always, because if you have alt, naturally you can glide, or reduce throttle while descending, and naturally if you're climbing you're flying slower: once you level yourself off you'll gain a lot of speed and the "range" reading will go way up.
P.S. One sector is 25 miles wide on the AH map. I personally think 30 minute of flight time on full throttle is good (if not more for longer flights) and anything less like 20 minutes full throttle is too little fuel for a good flight (consisting of taking off, climbing out, finding a fight, getting into the fight, fighting, flying out, landing). Also, having extra gas means running WEP a bit more freely (as this usually increases fuel consumption a lot).