What type of aircraft are you talking about?
If you're operating a piston aircraft, where the Climb / Descent / Approach sector corrections are quite predictable, and over a short distance, say 200 to 300 miles where the mean wind would suffice, then the conventional formulae should be OK.
If you're operating a Jet aircraft, where the Climb / Descent / Approach sector corrections are highly variable with Weight / Temperature / Cruise Level, and over long distances (e.g. trans Pacific), and considerable wind variation can be expected, then the only safe way is Wind Zone by Wind Zone all the way out to the zone in which the PNR will lie, and apply the standard formulae within the zone.
If you want to apply the standard simplified formulae in the last case, it's suggested that you review your ditching drills and practice your swimming proficiency
Regards,
Old Smokey