I'm always fiddling with being comfortable playing seated.
I like the idea of the BB King style sitting back in a chair; comfortable on my dodgy back, but cant manage it.
The angles just aren't right holding the guitar.
I find the biggest factor is seat height off the floor as it affects the angle the thigh makes to keep the guitar against my chest.
Yes, as Gasket says, the back thing becomes important very quickly. I find I can do the sofa sitting at the edge and angled left so the sofa isnt interfering with the guitar body. I try to keep my back mostly straight and bend a bit more at the waist to cradle the guitar body into my chest. The right chair with either the frame of the chair or some other footrest works well too. I have a mechanics rolling stool in the garage with adjustable height I been meaning to try.
The way the body and neck sit always seem too far behind me an my picking forearm is sometimes just not finding a good perch on the guitar body.
And sometimes I just get in a groove and everything is fine. I'm not thinking about it until the next time I'm not in that groove.
SG, Tele, Jaguar all are manageable but LP just sits too far behind me and trying to the left leg thing puts everything too far left. I mostly stand with the LP, though it is the heaviest of my stable. The leg cutout on LP and many others is very forward on the body.
The jag is the most comfortable to me; its something I noticed right away.
I never realized how much it matters until I started doing it.