higher the angle, the more shoulder you work. Lower, more chest of course.
Lower is gonna be more specific. I reccomend 30 degrees.
if the goal is to work the upper chest, 30 degrees is right too.
you need to remember that just as large arching effectively turns a flat bench into a decline bench, large arching can turn an incline bench into a flat bench.
especially if your goal is to work the upper chest, this is not ideal. only arch as much as you need to get proper thoracic extension and scapular retraction (just so you don't blow your shoulders, incline is especially tough on the shoulders).
an old bodybuilder trick is to put your feet up on a box at the end of the bench or something. flexing the hip more makes it harder to arch.
as the RPE goes up you will probably arch your back more in the middle of the set without even trying. trust me.
although for specificity sake you could argue a huge arch is more specific. but if you want specificity just do tegular bench or a flat variant desu. you don't always need to maximize specificity, and sometimes there is benefit in not doing so
and before some asshole like trappy chan comes along and says "aah the upper chest doesn't exist", there are different nerves that fire different groups of muscle fibers on the chest, and there are differences in EMG when pressing at different angles for different parts of the chest. there is also a ton of anecdotal experience with the lower chest being over developed from lots of pressing at a low angle, and then evening out development by doing more highly angled pressing.
there is also undeniably a clavicular head of the pec major.
also, pressing that has more shoulder extension may utilize the upper portions of the chest more, but for a different purpose and probably less effectively than a wide-gripped incline bench.
I should add that your grip on incline should probably be in just a smidge from your normal bench grip, and bring bar down just below clavicle.