This is 'anecdotal' but I can assure you that if you dont do 50% the pet takes half.

I have been charming from 44-51 on bloodgills in loio and my setup is thus:

Charm a bloodgill. Kill the other bloodgill (usually nuke once, or maybe not at all - never 50% of the dmg). Get exp.

Hope that the bloodgill i had charmed is <20% - Invis, and then nuke it.

The second bloodgill always gives me moveable exp / more exp in my bar than the second, purely because there was no pet to eat it. I have changed my strategy (nuke twice) and I get double the experience on the first mob as well, but then I usually need to find more mobs to kill my pet again. It's a vicious cycle.

Easy way around this is to break your pet when both are low, kill pet, and then kill the other mob. Results in 100% exp for both kills.

-Propo Fol

No, it's not anecdotal really. Especially going from lvl 1 to 60 with always having my XP bar up with screen shots.

Try this:
Get a Goblin Gazughi Ring and use it to break charms.
Use Cajoling Whispers, less mana and you do not need the charm to last so long.
Make a macro /pet back /pet attack /pet back /pet attack
Use the level 4 mesmerize after breaking pets to avoid a round of melee from unrooted pets or to force broken pets to flee by mezing rooted adds.

Charm a bloodgill. Agro a bunch of bloodgills with your pet, get them stacked up with your macro, root them with pe, start a timer for three minutes.
Once you got that rooted, if your pet is still alive agro more bloodgills, stack them on the previous stack, root them with pe, start a timer for three minutes.
By now your pet should be getting low on life, break charm when its a nuke from death and get that xp. You now have a stack of mobs to kill, find whatevers highest hp in the stack and charm that. Rinse repeat, remember to keep up with your roots. This works exceptionally well while you can still use Cajoling Whispers in areas you can also use theft of thought to clear lots of mobs with no downtime.

Master Enchanter droppin the knowledge bomb.

P.S. I miss you broseph

Edit: ooops

I am skeptical.

edit - skeptical of this working at all, let alone well, at the level where you want to be in LOIO. I didn't even have PE until I'd already made like 5 levels in LOIO. And this method is effective, but it doesn't even last forever in a zone like HS, at 55+, where you have c2 and tot and pe/fetter. I'd certainly be impressed if you felt like frapsing yourself doing this with the 8+ mob pulls that your post makes it sound like you did.