you are taught to get round a nav route safely and in the easiest manner possible, you work dont work out your g/s just your heading correction using MDR. because your using well defined points, memorising your route exactly (after leg 1 I turn right to 220 should cross a power lines and see a lake to my right etc) and your using these HAT chxs pre and post turning point. Simple, easy, safe fail proof. If your running late or fast you just recaluate your new ETA based on the what the time difference is , eg 30 secs late after first half of leg your gonna take an extra minute for the whole leg. As you have now excess power to catch up on lateness whats the point of working out GS. Never bust airspace in my life and don't plan to, and knowing how fast you are going will not stop you busting airspace, but more likely make you do it if you have your head down, working out a complicated formuale.
I am sure the big boys in the RAF using all those techniques you speak of, but for EFT its not needed.