Flameblast mana cost calculation

I noticed that increasing my mana regen isn't doing what I'd hoped with my flameblast infernalist.

- Level 25 Flameblast skill
- tooltip states 122 mana/sec to channel
- Cast time per stage down from 0.25s to 0.11s from cast speed
- 1400 mana pool
- 267/sec mana regen while moving, 224 while stationary


when I channel the skill it burns through my mana in 9 sec stationary, and 12 sec while moving, so regen isn't paused while channeling.

I thought that channeling cost doesn't multiply with cast speed, but even if it does the +120% cast speed that results in 0.11s cast time would cost 266 mana/sec right and I'd be able to sustain chanelling?


so it seems the actual cost of channeling this is 380 mana/sec? why so different from the tooltip cost?

The mana cost on the gem doesn't reflect the actual mana cost affected by support gems that increase the mana cost, to see the total mana cost hover over the ability in your hotkey bar to see the cost.