TWC = GS - TAS will only work if the wind is parallell to your direction of flight. More than that, and it is trig or vector calculations territory. Fortunately made easier to perform on the fly by advanced variations of the slide rule concept.
Some FMCs allow you to enter the predicted winds along the route to get more accurate predictions of fuel burn, top of descent etc.
Even when that is not an option, you should at least compare the actual times for each leg with the planned time, note any significant discrepancies and update your plan accordingly. If it is done, well... I’ll leave that to those who do it on a day to day basis.
Cheers,
Fred