If your bike is too rich, you would use more fuel. IMO 70-80 miles is pretty typical at a good trail pace with some fire roads.
Actual fuel mileage really depends on how and where you ride. Higher altitude = leaner jetting, more range. Hard terrain and fire roads will get you much better mileage, especially if you are cruising in top gear. If you are in the mud or sand and keeping the powervalve open all the time, you might only get 50 miles. Think about how much more energy you need to use if you push your bike through the mud versus a pushing it on a paved road. Same with your engine! Brenda rides in the southwest - hard terrain, better mileage.
On how you ride, a buddy of mine is extremely fast and he would always get better range than me when we had the same bikes, becuause he rides lower in the rpm range and is often a gear higher. He's going faster and getting better mileage due to his riding style.
Over 100 miles is certainly possible if you are not opening the power valve all the time and riding hard terrain. On my KDX250 I could go over 100 miles on dual sport rides, but trail riding it would do about 70-75. My '90 200 would usually get about the same mileage as the '91 250.