Confused about mana costs with indigon/incinerate

Edit: Just ignore this. I was treating the indigon stacks as a more multiplier. I have to add them to the fevered mind/hierophant additions and reductions. That probably means the mana cost won't grow as quickly.

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Hi all, I tried my hand at jamming as much mana/mana regen on a hierophant as I could afford to attempt to make an indigon/incinerate build, but it seems really lackluster. According to PoB, the base mana cost for incinerate before indigon takes effect is 30 mana (this includes checking always full ES to get the extra reduction from the hierophant node 'Sanctuary of Thought'). In game the tooltip reads 28, so I'm not sure which is right, but let's say for now they're reasonably close enough. Additionally, PoB has my cast rate at 10.5/s. So even without indigon, I should be spending 300 or more mana/second, no? And with indigon, each time I spend 200 mana I should be ramping up my mana spent per second in a positive feedback loop until I run out of mana. But this process takes a whopping 25 seconds.

I wrote a script to keep track of how much mana I would spend over 4 seconds (Since that's the time period the 'recently' keyword refers to) adjusted for indigon stacks (at each time step, calculate the total mana spent so far and adjust the mana cost to (baseCost * 1.53 * int(manaSpent/200)), where 1.53 is the multiplier for my particular indigon), and I calculated that I should start spending around a 1000 mana per tick after less than 3 seconds.

Basically, my question is 'Is one of my assumptions wrong?' Does incinerate not spend mana at every tick? Am I not understanding how indigon works correctly?