Seating Configuration
quality or quantity
There is no one formular for what seats to choose. You have to find the seating that best fits into the aircraft/aircrafts you use. You don't want to waste space, obviously. It also needs to have an approriate rating for the distance you want to travel. The better the rating, the more money you can ask off the pax, in turn, costs like fuel burn are going to be distributed over less pax if you choose better seating, so the per-seat-costs rise as well. In turn, the better the seats, the fewer there are to be filled, thus making it more likely to actually have a high load and operate flights profitably. As with everything, it's a trade-off.
seating by range
Since I hate micromanagement, I usually offer a service level for short-haul, medium-haul, long-haul and, should I ever introduce it, ultra-longhaul. Limits (at least to me, usually being 1000-1500, 3000-4000, 8000km.
Usually, I use the same seats on short haul and medium haul so that I can interchange aircrafts easily between those routes. For my Venezuelaen airline, I actually use a different seating for my national network, as I use Dash Q400s there and they don't fit my classical config used in E-Jets and B73x/A32X.
So, feel free to try around here. Since you asked for pointers, here is what I am currently operating on Gatow:
nationally: due to the lack of competition and since it better fits the Dash Q400, I am using leisure on economy class ( Y class or just Y ) and recliner shorthaul in business class ( C class).
regionally and midhaul, I am operating E-Jets with leisure plus in Y and recliner shorthaul in C. That has been my basic config for a long time and it works for me, as this also works in most B73X and thus increases inter-changeability.
seating config for long-haul differs as it depends on the a/c type and route profile you pick.
pax weight and range
every pax weights 95 kg, every CU 100kg. you rarely need the MTOW of an a/c. so take that into account when designing floor plans for the longhaul fleet. (You can check the weights and distances on the aircraft information page and use the performance tool to evaluate the available TOW vs. distance)
should the TTOW exceed the MTOW for the given distance, the first item to stay behind is CUs, followed by Y, C and F. if you really want to push the range of an a/c, you make a few hundred miles out of the 50 to 250 CU available in the bellies of longhaulers.
first class
for short-haul and even medium haul, I would not offer a first class ( F class). There is a demand and if you know how, you can make quite some money out of it, however, pax downgrade a class, if their preferred class is not available at all (so including all other available connections). The market is very small and I think it is not worth the trouble. Others will disagree on that one.
Personally, I don't even offer F on long-haul flights. Y and C work just fine for me. This also depends on the pricing model used on different game worlds as the standard prices vary.
Regarding distribution, I tend toward 15% C and 85% Y for short and medium haul, more C for longer flights. This, however, needs to be adjusted to demand in your country/region/on your route(s).