Distance from the speakers to the seating position

yep, all good info above.
Basically, it's all situational/system/room/gear dependent!
Some speakers need more distance from the listener. Some speakers can be placed further apart ingeneral, all things equal. While others need closer spacing, regardless.
I've come across it all. Alot has to do with, yes, crossover desging(gernerally 1st order neeeds more distance from drivers to listener, 4th is the opposite). Also, driver spacing and layout(many small 2 way monitors can let you liten closer often, while multi way, even large Diappolito's need more room to build sound coherently together). And yet, there are more varriables.
I often find, with two speakers only, yes, the triangle gets you in the ball park, at the very least, and you can tinker from there. Some speakers let you sit even closer however. So you must try if you like.
Another often MOSTLY OVERLOOKED ISSUE is room acoustics and set up!...as hinted at above earlier by others.
With a low ceililng situation, or where you're sitting back further, and or you are hearing more reflected sound mixed with direct sound from your speakers, you will be dealing with acoustic issues that will blur and smear your imaging, detail, dynamics, soundstage, and coherency! While in the opposite scenario, if you sit proportionately closer to your speakers, in relation to your room reflection points(walls/floor/ceiling), and thus hear more direct sound than reflective sound mixed with the original signal from the speakers, you will get tremendously better results in the afore mentioned areas!(imaging,focus, detail, dynamics, soundstage, coherency, etc). In fact, many people, especially those who use traditional "tweeter over mid/bass driver" design stereo speakers, are much much much better off if they can either strongly deal with their acoustical reflections in their rooms(thus allowing the original signal from the seakers to be heard with more purity), or SIT CLOSER TO THE SPEAKERS!...all things equal. Other more closed "focused designed" speakers offer more flexibility often.
Still, how far that can go depends on speaker design paramaters, and what the manufacture deems is appropriate distancing. It's a balancing act bsically.
So, if I'm using speakers that are designed(most 1st order designs need you to sit further back, depnding on driver spacing...my old Electa Amators had drivers sitting right on top of each other, so it didn't matter) to make you sit further back, you must definitely consider acoustical treatments/reflection points much more critically if you can't sit closer! With 4th order crossover designs, this often doesn't matter as much, and you can sit closer to your speakers, as with other designs.
Basically, you gotta tinker. Good luck

Each room/setup/layout/gear selection is different, and calls for different variables to be adressed for best "balance". It's all a balancing act, and all things must be considered together for the overall effect. You simply can't just take a blanketed formula, like an "equilateral triangle setup", or 10/11 whatever, and assume good restults! It's all very situational.
You must tinker and try things.
First thing I always do is FIGURE OUT THE BEST SEATING POSSITION(S)! I find the location(s) where I'm going to get potentially the best frequency response and foundation from with wich to anchor my EARS! If I can get smoothest response from there, I go there...all things considered. Obiously there's compromises often...but then you adjust from there too from the speaker and acoustic treatment end!
Anyway, I then am going to find the coresponding speaker locations up front that offer the best, smoothest, most even frequency response in relation to the seating position(s)! If from the seating position(s) I can get the speakers to yield the flattest and most even response, I've got a solid foundation for accurate, dynamic, natural sound(not boomy, lean, peaky, inaccurate pressentations). If I can't get that, I'll consider EQing the bass woofers if possible to deal with any serious challenges. Yet, I'll most always get the locations for both the speakers and seats working together to give flat response...CRITICAL!
You then must balance those "ideal" or "best" location choices with the soundstage and imaging parameters.(i.e, triangle, width, depth, whatever). If my location considerations for speakers/seats let me chose spots that will yield a setup that has the proper perspective and geometry for a sharp, coherent, solid image, with a good soundtage(width, depth, height, perspective, etc), then you're there!!! Then you need to consdier "toe-in" for tonality and focus fine tuning, as well as "aim" or "angle" of speakers towards listening possition", etc.
After that, I'm consdering all the acoustical needs, in relation to the set up(speakers and listening possitios). I deal with overall reverb(must study this part), and mostly treating corners for bass/imaging, and FIRST ORDER REFLECTION POINTS PRIMARILY! Once you balance out this part for best absorption/diffusion considerations, depending on room needs(long wall setups can get away with less of this often, while short wall setups usually require much more strongly considered acoustical add on's for good sound!!!!...due to signal reflection/boundary interaction to your ears!...mixing the rooms reflections with original signal).
Other than, in smaller acoustical spaces(most homes sadely), and then employing large bass traps there in, there's really not much more to consider in an average home audio set up! Maybe phase with a sub(s), etc, but not much else!
YOU SIMPLY CANNOT GO WRONG WITH WHAT i JUST SUMMED UP!!!
A solid foundation for flat and even(between speakers) frequency response from speakers/seats, proper speaker to speaker to listener set up(triangle, whatever), proper speaker toe-in/aim, proper phase(if applicable), proper acoustical considerations and/or treatments, and you cannot lose!
Other than bass management and EQing of difficult bass mode situations when applicable, there's not much more. Anything else is trial and error, and season to BEST RESTULTS GIVEN YOUR GEAR AND ROOM!
Good luck