As luck would have it, Lightsday was just starting as I read this, making all the figures much more fun to interpret >.>
The values I got were as follows (No cure potency items and I've removed the Lightsday bonus if I got one.)
Healing Skill MND Amount Healed (Actual Result)
264 70 623 685 (Lightsday)
264 80 639 639 (Lightsday, no bonus)
264 92 656 722 (Lightsday)
264 95 662 622 (Lightsday, no bonus)
264 108 678 601 (Darksday)
279 80 644 644 (Lightsday, no bonus)
279 88 655 721 (Lightsday)
Which leads me to believe the formula is:
H = Healing Skill, M = Mind
Amount Healed = (H / 3) + (M * 1.5) + 430
This doesn't give 100% accurate results though (occasionally out by 1 or 2 HP) but I'm assuming any errors are due to rounding or something...
How does that compare to what you've come up with? Similar or...?
Side note: This suggests that +2 MND gives you the same benefit as +9 Healing Magic Skill (for Cure V).