I'd swear I've done Mijin Gakure testing in the past, but here's some more. As you can probably guess, this is probably the most annoying testing I've ever done. How far it goes will depend on my tolerance, and I'm already running out of it.
Method:
1) Test Server, reset JAs in MH
2) Teleport to Ronfarue
3) Mijin the nearest monster
4) Homepoint, repeat
1533 HP Elvaan Level 99 NIN/WAR with 12 STR merits and no equipment
Spoiler: show
Average of 771 damage, which is almost perfectly 50% HP. You'd expect 43~44% from an even distribution, but it isn't an even distribution. Even from eyeballing you can see that it's weighted heavily between 50% and 60% HP. I've alternated bolding on 10% groups as kind of a ghetto histogram in the spoiler to show this.
Proof of concept, went 99NIN/RDM and Converted myself down to 151 HP:
Spoiler: show
Small sample, but all below the previous one and still a mean of ~50% HP. Looks like it does depend on HP.
That's all for now. Open questions:
1) Exactly how does the damage work?
2) Is the damage affected by MAB?
3) What boost does Reacton Arm give?
4) Does 99 Nagi boost Mijin's damage as well as giving Reraise?
- Murgleis' potency increases as it levels up, but there's no note on the weapon itself. Perhaps Nagi could be the same.
Edit, more testing:
99 Nagi on 1533 HP NIN/WAR:
Spoiler: show
Average of 50.7% HP in damage. Doesn't look like it has changed.
99 Nagi on 151/1474 HP NIN/RDM:
Spoiler: show
No effect like "makes it behave as if you have max HP."