I say stack because you get 3 umbral hearts upon casting blizzard 4. Not stacking the effect itself.

Also no i didnt say f2 wasnt worth casting i said casting -too many- isnt worth it when UH allows for multiple flares and f2s potency decrease is enough to push us towards double flare (without convert) aoe rotations.

Though im looking for it im positive i saw the flare reduction costing 2\3 UHs which is where i based all my info from.

Cant find it atm still looking but going by your info

from what im seeing from our 15480 mp from UI into f3 itll be 15480- F2 1800 (mp remaining 13680) (1UH consumed) - F2 1800 (mp: 11880 ) (1 UH consumed) - Flare 4644 (mp 7236) (final UH consumed) - Flare(0 Mp).