50% is the rule, if the charmed pet is doing that much damage you're probably doing something wrong as a druid though. As with necros, stack up the dots...and if you suspect the pet is ahead, pull it off at ~20% and let the dots guarantee the final damage to be yours over 50%.
Charm breaks, its a fact of life... just keep your sow on (enchanters don't have that luxury unless they've got jboots), snare it(?) and recharm.
Rare to see charming being done... kiting/quadding is much more druid-esque from what I've seen
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