But as you use the fuel the mass of fuel decreases.
So does the original journey plot take this into account?
Or does it limit each hop distance to the original mass calculation?
It does take this into account, it assumes you do not refuel until the last scoopable star. I scoop at pretty much all stars I can and occasionally but rarely this will mean I can't make one of the jumps originally plotted. You then just have to open the GalMap and it re-plots the route. However, this is pretty rare in my experience.