I have a skyview with a 912 iS and i fitted red cubes. I now use the inbuilt ECU fuel flow figures since i need that figure to manage power with a constant speed prop running lean of peak (eco mode - power is a function of fuel flow not MAP) I haven't bothered to finally calibrate the red cubes and they are too slow to react compared to the CAN bus figure. I'm removing the red cubes during the annual.
I'm told that the CAN bus figure is accurate to within a few percent and that has been my experience - anyway, I always work on at least a 10% tolerance which for me is +- 11 litres because I don't believe I can accurately plan better than that anyway so the CAN bus figure is fine. Anything more accurate would be overkill.