New Fire Mage - questions about ignition / combustion

[drumsmani 47]

I recently rolled a fire mage who is still in the low levels. I was going through the guide and have difficulty understanding exactly how the Mastery: Ignite works (by itself and with combustion).

 

How is this 4-sec tick component calculated? Is it based on the direct damage from the last spell that triggers Ignite or is it a total of the damage from the last cast of all the applicable spells?

 

Taking some made-up numbers (and base mastery), let's say a cast of Fireball causes 1k direct damage, will the ignite DoT be 120 spread over 4 secs (30/sec)? If I follow this up with an Inferno Blast that deals 500 damage, will the new Ignite value be 90 (remaining ignite) + 60 (from the IB) = 150 over 4 secs and so on? 

 

I also didn't quite understand the need for /stopcasting with combustion while queuing fireball and/or casting combustion before PoM Pyroblast reaches the target. If the ignite value is always going to be refreshed with the remaining amount why do we need these tricks? 

 

I am sure I am missing something obvious, I just don't know what it is. 

[Akomagus 8]

I'm not a math guy so I can't answer the first part but I know you need to stop cast to do combustion because while casting a fireball the ignite could drop drastically so you want to make sure when you combust that it's the actual level you want, a delay could result in a lower ignite.

[drumsmani 47]

Ok, I can see why I would need to stopcasting fireball if the ignite value reaches a threshold, since fireball has a casting time and ignite could tick before it finishes casting. Maybe same argument applies to PoM + Pyro since that has a travel time component during which ignite could tick. :S

[Vladamyr 169]

http://lhiveras-library.com/compendium/ignite

Hope that clears it up for you 

[drumsmani 47]

Oh, that's perfect! Thanks.