MDR seems accurate enough at 420kts
Anyhow, with a computer you are working out a very precise figure with a smaller tollerence than you can fly within and all on a met mans guestemate! Whilst the calculation is accurate you need accurate info to start with. Its often much MORE accurate to have an idea before you fly and amend your MDR as you see fit in the air as the wind changes.
Always comes out as 2 degrees anyway!
(I do however feel I am preaching to the converted, all bar one here though!)