Hi Elennoko.
If you read the destruction guide thoroughly, you'd see that
Grimoire of Sacrifice (GoSac) favours Mastery over haste and crit, which favour
Grimoire of Supremacy (GoSup) instead. This is because GoSac shifts your damage from pet and incinerates to chaos bolts and shadowburns. You're in a weird mix of the two that doesn't work out too well.
As for your rotation:
Understand that you do not use Rain of Fire for the sake of the actual raw damage it delivers, but as a boost to your ember regen, to be able to deliver a higher amount of chaos bolts. Also, because of the high mana cost you cannot (like you discovered) keep casting it all the time. Personally I've discovered that it is fine to cast it whenever I'm over
50-60% (
depending on if I have haste procs or not - the more haste, the lower I can put that mana-threshold) mana, but below that is only for special occasions which are as follows:
a) I have a high haste proc, for instance legendary meta or Bloodlust = faster mana regen through
Chaotic Energy.
b) the boss is below 20%, meaning I can chain shadowburns to give me enough mana to afford the rain of fire spam.
c) i'm going to cast 3-5 chaos bolts in a row = no spells that cost mana = mana isn't an issue for the duration of those casts = 100% uptime on Rain of Fire for that duration.