There are a lot of claims on what makes a perfect posture, from how far you are away from your screen to the angle of your chair.
However, there is not a lot of research to support that ergonomic guidance has a significant effect on back pain. There is even less evidence to show that one position is superior to others when it comes to sitting.
Ergonomic guidelines often suggest there are optimal ways to set up your chair, desk, and screen. However, the most important tip to remember is to not stay in any one position for a great length of time. Your should prioritise taking breaks when possible and shift positions when the position you are in starts feeling uncomfortable.
Exercises can be a very good way to help break up a long day sitting in a chair. This allows you to not only have a break, but increases movement and mobility. You may suggest to the rest of your coworkers a health challenge of sorts; this could be doing a plank or a certain amount of squats every day! It creates some accountability and makes it easier to remember to move during a long day at the office! Exercising and moving when you are able to will also have some other positive health benefits which may decrease some of the negative health effects associated with being sedentary.
There is not one position or posture for sitting that is superior to others. You may find a certain position more comfortable for you, and you are probably going to be spending more time in that one, which is okay. But after a while, most positions will start to be uncomfortable. At that point, simply shift positions or get up and take a short break.
Remember that the next posture is the best posture, and there is always some way you can change how you are sitting. The only bad posture, per se, is the one you spend too much time in.