I played around with Healing Magic/MND with Cure II and it seems:
199MND + 311Healing Magic gave me a low of 373, an average of 392 and a crit of 450
193MND + 301Healing Magic gave me a low of 362, an average of 374 and a crit of 460
179MND + 306Healing Magic gave me a low of 362, an average of 370 and a crit of 450
173MND + 296Healing Magic gave me a low of 348, an average of 360 and a crit of 440
So it appears that the higher the Healing Magic, the higher the floor; the higher the MND the higher the crit and potency (in general). However I have reason to believe that the recipient stats effect the potency as well. Casting with MND179 + 306Healing Magic on a level 17 Gladiator gave me a low of 346, an average of 350 and couldn't get a crit. No idea what stat (or maybe the level itself) is the culprit though, but it may be like FFXI's Dancer with user's MND versus recipients VIT...