More specifically, how does the base stat mana regen work?
Demigod Forums
The topic title says it all. I was just trying to do some math for my guide and I noticed that Rook starts with a 2.77 mana regen. Does this mean he regains 2.77 mana a second or is it based off a percentage of his full mana?
It is a flat value, so 2.77 means he gains 2.77 mana per second. There are items that give a flat mps boost, and items that give a percentage boost. It usually makes sense to get a flat +mps item, and following it up with a +% item for obvious reasons 
Edit to add:
Mana regen rates increase naturally as you level up, but they are never a percentage of your max mana. If you have 2k mana and a 5mps regen, getting an item to bring you up to 3k mana will still keep your 5mps regen.
+% mana regen works on +mana/sec items too? I thought it only worked off the base (and is thus useless)
Nope, it works off the total regen value, not base.
Well if your total regen is 31mps:
+15 = 46
+50% = 46.5
So anything over 30 is benefitial.
Another example. Rook mana regeneration at level 20 is 8.242.
Vlemish Faceguard gives +11 and +4 mana per second. Combined with a level 20 Rook that's 23.242.
Plenor Crown gives +70%. 70% of 8.242 is 5.7694. That gives you a total of 14.0114 mana regeration.
High percentages are misleading. They're only effective when your base mana regeneration is high.
The math changes which each Demigod as each one has different base and raising mana regeneration rates.
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