[Mythical Healing Potion] is limited at 80 or higher lvl to use but there is a workaround.
Using [Engineering: Spinal Healing Injector] on your gloves will let u use it at lvl 75-79. On my lvl 77 rogue with 17k hp it healed me for 16k hp(in theory it should have healed me for much more, maybe a lvl 78-79 with 30-40k hp can test this and report if it scales with hp).
Now this might be intended but considering the hp pools at that level it is very improbable that eng is meant to heal you for nearly full hp.
edit: should work with mana version too i guess