Nice work, that global illumination and ambient occlusion is working out really well for you. Nice balance of colors and palette. All the assets tie together nicely.
It's a percentage of the global bounding box of the object. since your chamfers are pretty big compared to the cube, you need to set this to 0.1 (10% of the size of the object) indeed.
Yes, I read an interesting article from the Substance team about the future of this. Autodesk are firmly behind this and we're not too far off a MatX/USD global standard.