Flight plans are VERY accurate.
Arfur Dent has it right! Ive been doing the long haul thing for almost 30 years. ALL of the flight plans i have worked with, have a TOC point, and there after for every point there after. sometimes every frickin minute!
I have seen flight plans predicated on weights minus a decimal point. I have seen flight plans and W&B predicated on kilos instead of pounds and vice-versa. I have seen flight plans predicated on a certain arrivals then to a point somewhere in space and another arrival to the same runway,
Computers are great! but................. sh!t in, sh!t out.
slow down, its your ticket, you are the Captain; or to be.