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The ambient pressure at a 12,500 foot DA is 9.17 PSI. Subtract 4 from that and you get 5.17 PSI which puts you at FL260. A 5 PSI delta will get you to FL310.
The attached ICAO Atmosphere table is from the LEE Handbook and shows the corresponding altitude for a given pressure.
I prefer to run a 5 PSID at all times to reduce fatigue and improve mental acuity.
Regards
Brent Regan
ICAO Atmosphere.jpg
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