I'm gonna stick this here just to remind some people that this will never be "fixed".

• 40y Range (Heal Spells) in the World (non Instances) & Battlegrounds is 99% accurate
  (It is triangulated from map coordinates, which means height differences throw it off a bit)
• 30y Range is always 100% accurate since it uses an interface function.
• 40y in Instances & Raids is only possible through the combat log or by targeting the specific player
• Combat log range might be altered by other addons this why Combatlog Range checking is off by default
• You can't target 40 people every 1 or 2 seconds, the ui would simply freeze
• What works is if "Auto self cast" is off in the interface options. It allows to have a spell on the cursor (blue glow). As long as a healing spell is on the cursor 40y range will work 100% accurate.

Not spending any more effort into the legacy version.