that was one of the more annoying features of the 700 fuel gauge, it goes directly from 2 black bars (plenty of fuel) to flashing one bar (panic, running out). That all changed at the 750.
As for fuel used, mine starts flashing at a predictable and repeatable 10.5 lts used (fill to just above the baffle, bike on the centre stand).. Best thing is to fill up as soon as you possibly can after the flash starts and see how much you put in. After a couple of times you should fid out how consistent it is, and what your typical miles/litre usage is. Then you can assess how many miles you c