A corroborating reference to Mr. Rich Ay's above remark can be found in WRC 271, entitled "Gasket Leakage Behavior Trends: Results of 1977-79 PVRC Exploratory Gasket Test Program." In this 1981 Bulletin, for spiral-wound, asbestos-filled gaskets it lists a stiffness of 471,000 psi/in for 300-lb class, and 695,000 psi/in for 600-ln class. No one uses asbestos anymore, and compressed asbestos is a hard material.
CAESAR II's suggested numbers of 347,000 t0 437,000 sound about right.