No angle change at all. Just cut the first cut about 1/4" too long. Final cut in one pass with circular saw. Maybe you could cut it 1" too long then try a full cut pass as a test run before you cut the actual exact cut.
I didn't pick up on you finishing the slabs before fabricating the corner Joint & cut out. Just be aware you will probably need to sand the exposed faces to get a perfectly smooth surface. All glue joint surfaces need to be unfinished so the epoxy can soak in.
That sycamore has been cut off a river bank or a low wet area to have ambrosia streaks. The ambrosia maple the same way, needs to be around water to have the ambrosia streaks. If you cut them on the side of the mtn they will be white with no ambrosia streaks. Box Elder the same way to.