Playing black mage is something I take pretty seriously and rotation is something that I often make fine tweeks to.
Honestly, no single guide is going to give you all the information you need.
There are simply too many factors to consider. Things like "wait till x mana" or "cast thunder II and not thunder III" are just simply not a good blanket.
The rotation I use now at level 50 (i86) is not the rotation I used when I was 50 (i55). For example, pre 2.1, I did user thunder II after my blizzard III,
followed by Fire III to begin my fire rotation. I can't do that anymore. With the increase in spell speed, I can Blizzard III and have enough time to Thunder III,
and STILL not have max MP by the time my Fire III goes off. Depending on party composition, sometimes I can slip a Blizzard I after my Thunder III before heading
back into astral fire. Maybe a scathe if I'm on the run. It's all about situational awareness more than anything.
See more at How to Maximize Black Mage DPS in FFXIV ARR Patch 2.1