As for the angles, I usually start as low as I can, raise the spine a bit until I end with a raised burr, then the same at the other side. The end result will be an edge of some 10 degree at the right side, and some 15 left.
With some blades and some usage such an edge will need a microbevel.
I guess I wanted to go with something more than trial and error. For example during my last sharpening session I decided to go about 10-12* on my shun. I was able to pop hairs and shred magazine paper with little effort. After about an hour of prep work on a poly board the edge was in horrible shape. It rolled on me, started to chip and overall looked like it was in a fight.
Next time I will need to go back to 16-20* since that seems to hold up better. I don't know if that is because its a shun or if that's because VG10 should not be pushed that steep.
At the same time I put the same angle on my Carter but it held up much better. I still need to put it through a proper prep session on that poly board. If it does hold up dthen I'll continue sharpening it at that angle.
Next up is 51200, I'm going to go steep and see how it holds up. I have a feeling I might need to keep it in the mid-teens. Suggestions?
About your Shun: it has both to do with Shun and with the poly board.
But in general I would avoid such drastic changes. I prefer finding a solution with microbevels rather than change the entire geometry and having to -- partially -- undo that again. It's a waste of material.