This is why I like having our own UK forum, as my manual and experiences don't match up with the blurb on VSRI. Plus for the K7 650s, the goalposts have moved somewhat.
My gauge seems very accurate. I normally do about 190 miles with three bars left - that's about half way on the gauge. To fill it takes about 12 litres - that's about half the 22 litre capacity.
Furthest I've done so far is probably about 250 miles with two solid bars left, so I think 300 miles on 20 litres or so should be easy.
You DL1000 boys are just jealous.