I agree, to fly a J230 at 100kt is not good (unless other influences force this such as turbulance, traffic etc). It will be well below the recommended cruise. There are plenty of other discussion topics here about the damages it causes running engines at lower cruise revs to save fuel. I would think 25 to 26 ltrs an hour is about right for 2850 to 2900 rpm which should give you close if not spot on your 120kt cruise.