A fire crit is 150% damage. When you have the Ignite talent, your fire crits will deal 210% damage theoretically, as you get 40% of your damage dealt as a DoT. 40% of 150 is 60. Add the 60% to 150% and you get the 210% damage. That in mind, this is what ignite says:
Your critical strikes from Fire damage spells cause the target to burn for an additional 40% of your spell’s damage over 4 sec.
Here are some things that are not stated:
-If you crit again before ignite finished dealing all of it’s damage, a stack of ignite will be added and the dot timer will be reset.
-Ignite will stack up to 5 times.
-Ignite will tick twice, at t=2s and t=4s. 20% of the crit’s damage each tick.
-The total damage from ignite is credited to the mage with the highest ignite-generating crit on the mob at any given time.
-This mage will lose credit when that ignite stack completes its damage OR when another higher damage crit is applied to the mob.
-If a new crit is applied to 5 stacks of ignite, that crit will reset the ignite timer and replace either the oldest or lowest damage stack of ignite on the mob, unclear which.
-It is possible for a fireball crit with one tick already completed to be worth more than a fresh scorch crit, thus opening up a small chance that one fireball crit can earn one mage 6 seconds of ignite credit rather than 4 iff the concurrent scorch crits are on the low end of the damage range.