I'd say if you're struggling to write those next (plot-wise) chapters, jumping ahead is a good idea. If you're excited to write the last 20K, then go to it. Sometimes leaping forward to the part your brain is ready to write is a more fruitful process than trying to force the in-between.I say this as someone who used to only write in order and now hops around. I usually jump forward to a turning point or where two characters meet/have a significant emotional moment (big fight scene, big kiss, etc.) and find the connective scenes between where I was in the book and that point come much easier afterward OR I decide that maybe the scene(s) I'd planned there don't quite work and that was why I was stalling out.