I know that the %age varies between 1 and 3%, but I don't know what determines which one it is. It could be based on army size, mana taken etc, or it could just be a random integer between 1 and 3, inclusive.

By searching about it that's what I read but I found nowhere more details. I noticed a -2% but no idea from what it comes if it's not random, that's clearly not the fight difficulty. 100 fights is a lot, even something around 60 is a lot. I'll try it a bit more and will throw it if the percent doesn't decrease faster.

I have found it in the undead ship you have to summon out of the pirate seas. It would be a lot more difficult to level later in the game; I have ignored it in those instances. On a lower difficulty you could probably get away with using it against armies you match up well against.

Yes I got it in previous game and always gave up using it. In my current game it's first time I get it sooner so I tried use it and get a bit disappointed how slow it was to go to dead skull.

Well it seems that now I get more % higher than 1 and I have to admit that the mana decrease isn't a burden yet with tactics I'm using, at worse I had to use some magic spring but that was rarely a bad choice.

I found it in the same place I've always found it, the Necromancer house, but have never given it a try until now with my Paladin; the half mana always turned me off.

Just started going to the Elven lands, level 17, the first fight dropped 3% off it. I don't use mana for much other than Resurrection with the occasional Mass Battle Cry, Blind, or Haste. We'll see how it goes.

You can find it very early in the game (if you are very lucky that is) at the graveyard castle in the Marshan swamps. My fighter found it (typical!)

I found it somewhere in mid-game (I think--I have not yet finished the game) and have been using it consistently, even in the fight with Karador. It's now down to about 12% and I can't wait to see what happens when it's at zero--if I can win the next couple of battles. I think it goes down at about 3%, at least for the larger kind of fights I've been in since I had it. It will be very nice to get full mana back.