IO540 - try using "max drift is half the wind" in a 200kt aeroplane and you'll find it isn't accurate enough. Airbedane's formula is a better one to use.
A slide rule can calculate correct to three decimal places, so there's no issue with its actual accuracy. You should be able to fly a heading correct to plus or minus a degree or so. If you use an appropriate wind-correction technique, it will negate the inaccuracy of the wind forecast. If you get it right, you'll arrive directly overhead your destination.
This reminds me of the old story -
(NAVIGATOR) "Navigator to Pilot - turn right one degree."
(PILOT) "Sod off!"
(NAVIGATOR) "Navigator to Pilot - turn left four degrees."
(PILOT) "Left four degrees, Roger."
...short pause...
(NAVIGATOR) "Navigator to Pilot - turn right five degrees."
(PILOT) "Right five degrees, Roger"