The realism of seating arrangements in this game really comes down to how much work you want to put into it. I have split up my ideas and "research" into three points about what to do/take into account when trying to make a realistic seating arrangement. The first point covers the easiest way to create a "realistic" seating arrangement that fits the average route demand. The second and third points cover the analysis of what real airlines do when it comes to configurations.
1.) I studied the demand on a bunch of routes and determined that a demand-fitting configuration should be ~2% First, ~12% Business and ~86% Economy. The only problem is that each class has a different size of seat (In multiplicity of size of economy class seat: F=~2.5x C=~1.7x) and therefore requires a different balance in order to achieve the "perfect" seating arrangement. When it comes to having a realistic number of seats in any given aircraft, you simply reduce the seats of each class to 70-85% of the original config created from the 2%/12%/86%. Using the example aircraft, the B737-800, a high density demand-fitting arrangement would be 3F/21C/147Y and a lower density/more realistic seating arrangement would be 2F/17C/119Y (81%). Applied to wide-body aircraft the idea stays the same with one option: to become more competitive with other airlines using high-density arrangements on long range routes you can optionally reduce the seating by 5-10% more in order to allow higher weekly frequencies. Ex. High density for A330-300 is 8F/48C/343Y and low density is 6F/36C/259Y (75%)
2.) I recently studied the arrangements of various carriers operating the B777-300ER such as Asiana, Singapore, Garuda, EVA, etc. and came to the same conclusion that you did, everyone has a very different arrangement. Most airlines I researched also had several very different arrangements for the same aircraft (ANA has 4). I believe that any arrangement that is between 50% and 75% of maximum capacity is realistic as that tends to be the range that most airlines have.
3.) I personally include first class on almost all of my aircraft that will be operating on routes with first class demand. I have seen that most players in this game don't have first class on all of their arrangements but those of their largest and longest range aircraft. Most major airlines seem to have a similar approach to what most players of this game have when it comes to this. I am pointing out that another aspect of "realism" that should be taken into account when wanting to create the perfect "most realistic" seating arrangement is the inclusion of first and business class based on the overall size of the aircraft or intention of use in service.
I am planning to create a spreadsheet of various different configurations for the mainline aircraft of the game if you are interested, or anyone, for that matter.