Mana Leech gem weirdness

I've been building my spark witch (spark w/multi projectile, faster projectile, piercing, mana leech and some other supports and projectile weakness applied often to encourage piercing) over the last week, and hadn't noticed much help from mana leech, if any at all, but I left it slotted because I couldn't find anything else I wanted in the slot.

This changed radically very recently, after I hit level 51 and my mana leech gem reached level 4 (still only 1% leech). I have been trying to determine why I am suddenly getting so much more leech than I was accustomed to, so I tried a few things.

- I tried replacing my level 4 mana leech gem with a level 1 mana leech gem, thinking there was a display bug with x% leeched. There was no noticeable difference between the level 1 and 4 gems.

- I also had the gem slotted in a different off-hand item before I leveled. I tried changing it back, thinking the gem wasn't activating properly in the previous item for some reason, and had started working in the new one. This test also did not cause any noticeable difference.

- I gained +4% lightning damage from my new passive. I tried removing a piece of spell damage gear to cancel out this change, but still noticed increased mana leeching.

Now, I read a little on the forums about mana leech, and discovered that if the amount of mana leeched per hit is less than 1, then no amount is leeched for that hit. But lightning has a very large range, as far as damage is concerned. The change to my mana leech was like going from night to day. If it is based off of damage for each hit, then with 1% mana leech, I should have been leeching mana off of every hit above 100 damage, correct? This does not seem to mesh with what I have observed from mana leech so far. Is the average damage of the skill the value that is used for determining amount leeched, possibly? Some plateau effect from rounding might account for some of the massive increase I am seeing. But removing some of my spell damage gear should have canceled that, unless it is not being factored into leech damage properly.

When I say that I am seeing a difference, I mean that I used to have problems keeping up with chugging pots constantly, especially in target-rich environments, even though my sparks have a very high chance to pierce (with projectile weakness applied) and literally flood small rooms and hit half a dozen times per spark easily when they are very crowded with high hp mobs. And they only cost about 50 mana for 3 projectiles per cast.

Ever since this change, I RARELY drink mana pots in indoor areas. They just aren't needed anymore. It's an INCREDIBLY drastic change, and I can't seem to figure out why.

Even if I am right on the average damage thing, I should have noticed this as soon as my average spell damage went above 100, in theory, and not the 500+ where it currently resides. I don't really have any other ideas here.

Ah ha. Yes, the off-hand item I changed to does have Increased Mana Regeneration while the old one does not. That must be the culprit.

It's good to know I'm not just imagining this sort of thing.

Edit: yeah, I did run a test with removing the item, but I also had another item I replaced while in town which I had forgotten about that had Increased Mana Regen on it.