Octane requirement

Ive been running 87E10 in my 360 at 10.7 Scr/8.6Dcr, at 900ft, for over 100,000 miles with Zero problems. Now, it does have aluminum heads, which they say sucks about a half to .8 of a point of compression. So that might be a 10.2/9.9Scr equivalent.
At a true 9.0Scr,and at your altitude,your Dcr is so far down, you should be able to run the cheapest skunk-piss out there.
However, if you really want to know, and you cannot hear detonation, this is what I would do: I would get one of those windshield mounted accelerometers, that spits out horsepower graphs. Follow the directions in the box. Forget the disappointing numbers that it spits out. Look at the graph it generates.Capture it, to a paper copy. Repeat the test with 2 or 3 degrees less power timing. Again capture the graph. Run the tests several times to verify repeatability. Then start adding timing, 2* atta time. Do not exceed 38*. Label all the graphs as they are generated. Compare them. The one with the highest horsepower, and the smoothest line is the sweetheart.This takes care of the power-timing.
Next you can use the same tool to dial in your WOT jetting. If you lean out the mains for best power, then you should revisit the power-timing.