I have been planning to do a 747-8 flight (passenger) from Seoul to Los Angeles but I do not know how much fuel I need. I had past experiences where I had enough fuel but once I got to cruising altitude, the ETA shows red, even though I had up to 3 hours fuel reserve. What should I do? Note that this specifically happens on the 747.
AndrewWu has made a terrific calculator and guide for fuel burn, you should be able to make a copy of the google sheet and enter in the information of your flight and it will give you actual fuel for IFs fuel burn
Actually the fuel amount that Simbrief displays is perfectly right, the estimated flight time what you get from when you are fueling up your plane on ground is wrong. So just rely on the fuel amouth that Simbrief gives!