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Rough math here...really rough.
Std temp is 15C. Subtract 2C/ 1000ft.
90+15=105/ 2= 53,000ft roughly.

That's with out a calculator, METS, so forgive me if I'm way off.

Way Up there about 9-10 miles High!

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By checking the Skew-T log P diagram of the San Andreas Islands you will see that the -90 level would correspond to 16.6 km in this case, 10.3 miles, around 96 mbar of air pressure.
Not so rough after all