I am not a very high ranked player so I don't think my opinion would matter much so I will just drop what I have experienced.
I farm OD boss (normal) and my fuel cap is 66 as of now. (lvl 95)
I get to farm the boss 6 times.
Now, I normally get 1 fortune box every 60 fuel I spend farming and I would say it has the drop percentage of 90%.
When i get lucky I get 2 FBs' in the same 60 fuel trip and that has the chances of like 10%.
Hope this info helps.