New BLM, rotations questions and advice

[TimeDilation]

It hits for 450. What does your fire III hit for?
Even if it's more, Flare hits multiple targets for that damage and this is about AoE usage. It doesn't drain your mana under Umbral Ice so you'll get another Flare doing the Astral damage right after.
So yea, your flare hits 3 targets for 1350 and then you get a 1200 x 3 flare right after. It only replaces Fire III to convert your stacks but has higher base potency and an aoe effect.

If you choose to use a glitch is up to your own discretion, but the dps is better.

Fire 2 deals more damage than the glitched flares. Please parse your dps first than spreading this myth that glitching chain flares does the most aoe dps. It doesnt.

[Allyrion]

Fire 2 deals more damage than the glitched flares. Please parse your dps first than spreading this myth that glitching chain flares does the most aoe dps. It doesnt.

You're still comparing the wrong spells....
Also, only the Flare that converts your Umbral stacks to Fire is a glitch because it doesn't drain your mana. Quick casting a second spell is not a glitch. I hope you're not under that impression since you're even talking about Fire II.
Fire III(instead of Flare) --> Flare is still better dps than Fire II.

[Dyechamp]

Single Target

ThunderII > Fire III > Fire + Procs until 600ish mana > Blizard III > Thunder II >Fire III > Fire III (speed proc from astral IceIII) > repeat with fire

AOE
Fire III > Raging strikes >Fire II > Fire II > Fire II > Flare > Convert > Swiftcast > Flare > Transpose > Blizzard II > Blizzard II > repeat without double flare

both of these rotations have worked great for me.