You don’t need to go all the way down to reserve to know how far you can get before needing reserve.
Next time you fill up, reset your trip odometer (I do this everytime, my trip odo is my fuel gauge).
Go for a good long ride (or several rides) in typical conditions for you (if you’re normally heavy on the gas, stay heavy on the gas, if you normally visit route with lots of hills, go on a route with lots of hills).
Once you’ve got a good amount of miles on your odometere (you want to use in the realm of half your tank).
Then, when you’ve fill up, check how many gallons it took. If it used 2 gallons for 100 miles, then A) you can brag about your 50mpg motorcycle and 
you know with a 4.2 gallon tank you can easily cruise for 150 miles before worrying about looking for a gas station (that leaves you roughly a 1/4 tank left).
Me, I find I’m getting about 60mpg on my 250. I usually fill up at 200km because it is convinent (I’m still at half a tank at that point).
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Awesome to hear about the mountain ride with the streams. I went for a ride in the local hills last evening. Bright sunny, long sweeping turns, and some specatcular views. Such a treat.
