Hi, as an airline pilot, I can agree that there are tons of factors to be considered, especially flying IFR and jet engines. One is performance: on very short flights the general rule is calculating a flight level as high as possible that leaves a section of five minutes at level flight in that cruise level, but as also mentioned the optimum flight level does change with weight, prevailing winds and temperature. The heavier the aircraft, the lower you fly. If you can get your hands on the performance handbook of your aircraft you have many, many tables you can use. However, a modern jet should be able to indicate its optimum cruise level to the pilot, especially if fed with good wind data for the different flight levels and the altitude of the tropopause.
Then comes the question of airspace structure. Most countries use that semicircular rule that was mentioned before, others (especially those that have a much bigger N-S extension than E-W) turn that semicircular rule by 90 degrees. Furthermore, using airways, that rule may once again be flipped on its head for certain airways. Here, you may want to consult each (!) of the overflown countries AIPs.
And then of course there’s always the question of the minimum enroute, offroute altitudes as well as the minimum usable flight levels (MUFL).
Modern briefing systems take care of all of these and are good in calculating a very reasonable sequence of flight levels (remember, as fuel is burnt, the weight decreases and the optimum rises), provided that the actual weight and weather match the planning conditions to a high degree.
How well Simbrief does that? No idea, I use MSFS only to fly VFR.
All in all: there is virtually no limit when it comes to perfecting flight planning and execution. Look for the best compromise for your needs and wishes when it comes to SIM-flying!
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