I had the 250R and 500R and it sucks that they don't have a fuel guage. that's why I switch to a 650R that has a fuel light. but either way, you will know how many miles you can get out of a tank of gas.
like the others have said, the trip meter will let you know if you're getting close to empty.
Nope, just use the 2nd trip to give you an idea how many miles you have left before hitting the reserve. You get an idea how much gas you have left once you get use to it. Even my ZX6R and ZX10R don't have a fuel gauge. Just a fuel light (same as hitting the reserve).