We have a kind of hand-me down sofa (bought it off my parents).
It has a bucket type back? You know the arm and back are the same height but it does go up a bit more in the back.
Anyhow I sit tucked into the corner with 1 or 2 pillows propped behind my back. And pop my feet on a low stool or my sewing basket.
I used to work in a Nursing home and one of my many jobs was fitting wheel chairs for people.
Your lower back support is crucial. As is having your feet up enough. For the majority of us we want our legs to bend up and rest only lightly on the edge of the seat, then slope away from the sofa toward the floor. NOT a 90 degree angle. I am nearly 6 ft tall and find it much better to have a stool beneath my feet!
And last but most important and already mentioned... you must change positions! Every half hour is enough for most of us as we are not elderly and have a little or more than a little natural padding.
That being said... we've all got a natural comfortable position. There is no need to fight what makes you comfortable!