How does one go about figuring out what to tweak and what number to tweak to?
Millz has provided us with many options we can tweak / change. In terms of basic changes you will play with as destruction are the following:
1) Minimum health the mob has to cast Immolate on
2) Minimum hp of mob to cast chaos bolt.
3) When to use Dark Soul: Instability
Here is what I would put as a general setup but it may change based on the fight. Think about the mechanics of the fight and what will work best for you. This also depends on gear. If something is going to die faster then 10 seconds, probably don't immolate it. If something is going to die before your chaos bolt goes off, don't cast chaos bolt
Rough guidelines - tweak as needed:
Immolate: Solo 500,000 hp minimum - 5 Man Heroic 1,500,000 hp minimum - SoO Normal 10 / LFR - 5,500,000 - SoO 25 Normal 10,000,000 - SoO 10 Heroic - 10,000,000 - SoO 25 Heroic - 15,000,000
Chaos Bolt: Solo 0 (no minimum) - 5 Man Heroic 700,000 - SoO Normal 10 / LFR - 3,000,000 - SoO 25 Normal 5,000,000 - SoO 10 Heroic - 4,000,000 - SoO 25 Heroic - 6,000,000
Dark Soul Instability: Think about the fight and when you want it to be used. If the setting is "Use On Boss" you will not cast dark soul until you're fighting a boss. On a lot of fights that is great BUT on fights like Galkaras you won't use dark soul until he comes down. On Spoils of Pandaria Dark Soul won't be used until a boss box is opened. Depending on what you want those may be the best options. I prefer to put "Use On Cooldown" on those fights. When you are solo grinding use on cooldown makes more sense as well.