Life/Mana Leech calculation question

How exactly is life and mana leech calculated? Specifically I'm referring to the Mana Leech support gem.

My Ice Shot does:

6-19 cold damage
24-43 fire damage
1-13 lightning damage
9-29 physical damage;

and I noticed that I was receiving zero mana back whilst shooting large packs of mobs with LMP and Mana Leech supporting Ice Shot in a 3-link item. This was using level 1 Mana Leech which is 1.0% leech.

If leech is calculated per instance of damage, I can do up to 43 fire damage which converts to 0.43 mana (1% of 43). Is this then rounded down to 0?

This would explain why I am getting no leeched mana. I am fairly sure this is the reason, after about an hour of testing and thinking I was going crazy, but would like confirmation if possible.