There were quite a few things bugging me so since there was nothing else to do in SRO, I tried them out...
Disclaimer: All tests were done on a lvl 80 Pure Str Warrior and a lvl 80 Pure Int Cleric. The tests was done while moving to avoid hp regeneration. Also, I'm still not sure if the recovery results are absolute or if they depend on your STR value (Note: Tried testing without armors on, on both chars, it made a slight difference, just slight...)
1. How much does an XL HP Potion heal for?
Ans: 7505 HP over 2 seconds.
Helpful hint: For pure str euro, if you use a 25% hp grain, at say lvl 80 with approximately 20k hp, you'll get a 5k hp instantaneous heal with a 3 seconds cool down.
2. What exactly does the option 25% HP Recovery do?
Ans: I've made several test with and w/o wings and found out that this option does not affect Potion or Heal skill effects but instead gives out 25% hp more regeneration when you're stationary. (FYI: When you stand still you regenerate a certain amount of hp every 3 secs, in my case 164 hp with angel suit off and 201 hp with it on.)
Helpful Hint: Duh, Why get it? Hehe!
3. What does Weapon Reflect Percentage do?
Ans: Well, this is where it gets complicated. It varies...
Pure Int test
Wpn: 144 Wpn Max Matk (Lvl 16+0 0% Matk Rod, can't properly test with a 72+5 61% matk Rod)
Skill: 5198 HP Healing Power / 300% Wpn Reflect
HP Healed: 5631 HP
Bonus Heal: 433 HP
Result: It closely matches 300% of 144, so it seems that the weapon mattk reflect holds true in a pure int's case.
Pure Str test
Wpn:1778 Wpn Max Matk (Lvl 72+4 41% Matk Rod)
Skill: 1774 HP Healing Power / 300% Wpn Reflect
HP Healed: 3758 HP
Bonus Heal: 2014 HP
Result: It does not match 300% of 1778. Instead closely matching the average value of my total matk, which was 1868~2241, that has a value of 2054.5
Feel Free to add your own results or challenge my results. Would be nice to finally figure them out. Add your own tests out of boredom too.
Edit: (Thanks to Mutiny's Suggestion of incorporating Mag Balance)
Weapon Reflected Bonus Formula:
Weapon's Average Magic Attack X Magical Balance % X Weapon Reflect % = HP Heal Bonus
This equation also explains why there was some heal differences when I took out Armors... Hehe! OMG, Finally... It all makes sense now!
Proofs are down below (Scroll Down)