I couldn't find the answer to this, maybe someone can help. How do magicka cost reduction enchantments stack with the perks for each school that cut the cost of spells in half? For example the incinerate spell costs about 300 mp. The perk would make it 150. If you have enchantments for 10% destruction cost reduction it would then be 135mp.
Or does the enchantment get added to the perk to make it 60% reduction which makes the spell cost 120?

Did I do the math right and does the question make sense?

Magika cost reduction formula:

spell cost = base cost * skill multiplier * perk multiplier * equipment multiplier * dual cast multiplier
skill multiplier = 1 - (skill/400)0.65

Thanks. I'm not awesome at math so it looks like it's a percentage of a percentage and the enchantments don't get added to the perk. Right?

In lay man's terms:
The perk takes off half of the magicka cost (150). The enchantment percentage deduction effects the remaining magicka cost after the perk reduction has taken effect.

So basically the destruction perks cut the spell cost in half. Then the enchantments take down the remaining spell cost regardless of the destruction perks.

Even more basically. Spell cost Enchantments dont stack with spell cost Perks.