duks: have you seen a test other than Sherman Bell's?
It's here about 1/3 down
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit
The average increase in his study was 700 psi and 1 load increase was 1200 psi