I appreciate your attempt to help, but what works for your body may not work for others - I'm looking for general ergonomic principles backed up by sound scientific evidence.
I can't provide you with the scientific basic for correct elbow angles and back erectitude, but I will echo the GP's meta-suggestion of 'variety'. I've heard you're supposed to change position every 20 minutes, and stay no more than 2 hours in any position. Walk around, swing your arms. This is the first-order bit. Most other techniques and tools seem to be band-aids to help you tolerate longer stretches of time in a single position. Which, do what you gotta do. But changing it up is the most important thing.