Logs:
You cannot compare logs like that. Those two logs you linked have multiple fights, with different mechanics and trash included in it. The only fair way to compare logs is when they are the same fight. For example my damage on Zon'ozz is much higher than my damage on Ultraxion because Zon'ozz takes 10% more damage after each bounce and Ultraxion does not ever take any additional damage.
http://www.worldoflogs.com/reports/rt-r ... target=200
You need to keep Living Bomb up more. It's okay for it to fall off for a few seconds or to wait on refreshing it to use Hot Streak procs for Combustion but it can't be off the boss for that long. It looks like over a minute that Living Bomb was not cast which is just too long. Also, try to use Flame Orb more often. You only used it 3 times in a 5 minute fight. Flame orb
Combustion:
Getting a large combustion is based on the size of the ignite and the presence of the Pyroblast! dot and Living Bomb. Most of the damage from Combustion is from Ignite which is why it's so important to use Combustion while
First of all does Ignite stack?
Yes it can, but doesn't "normally". There is not enough theory crafting out there to know for sure how to achieve this except that it requires two critical strikes to hit the boss within two seconds of each other. It's incredibly unpredictable, but should it occur and a large ignite is on the boss (like 30k+) it should be taken advantage of. This is one instance of where fire has high RNG.
Second thing for example shall I cast combustion after seeing 1 rly high crit immidietly?
Don't cast it when you see a big crit, but when Ignite ticks for a large amount.
What is high crit now for good combustion?
This scale is based on my gear, but it may apply to you.
0-15k Bad. Really bad.
16-19k only if you have to
20-23k is decent. This is a common ignite for me.
24k-26k strong
27-30k really strong
30-40k Ignite stacking and really really strong
41k+ Use combustion. Like now.
Keep in mind that bosses that take additional damage will cause the scale to change with it. It just takes some good judgement to decide what to take.
One more if I am trying to get high combustion while spamming FB using hot streak is wise or no?
Absolutely use Hot Streak procs. Most good ignites will come from a Hot Streak proc.
Gear:
What you are wearings looks fine. Your reforging doesn't really make sense to me. I would suggest you use Wowreforge.com for your reforging. Enter in these values:
Hit to at least 1742 (value over cap = 0)
Haste to at least 1505 (value over cap = 1.2)
Crit close to (any really big number like 3000)
This should help to make your reforging more optimal in terms of getting closer to the hit cap and obtaining more crit rating.
Hope it helps,
-Guromin