Its hard to say what ignite to use combustion on, as it is very personal(gear dependant) how high you will/can go. So it comes down to experience on your character, trial and error. It is also important to understand that different situations will give you a varied ignite for combustion, Ie on pull your ignites will be way higher than mid fight, because you have all your procs+pot 100% and often heroism as well.
If i would be forced to guess, i would say your pull ignite is maximum around 150k, and mid fight ignites properly shoot for something like 60k. But again its hard to say and you will learn with practice and tests(not dummy..) what's optimal for you. And also sometimes you just have to realize that you wont get your maximum ignite and take what you have, as delaying combustion for too long can net you with a loss of total combustions cast per fight.