I just received another hint for adjusting accelerations at different damping:

A draft document that may have not been issued - ANSI N 14.23 "Draft American Standard Design Basis for Resistance to Shock Vibration of Radioactive Material Packages Greater than One Ton in Truck Transport", American Insurance Association, May 1980 - lists a damping conversion that uses the power of 0.2 rather than 0.5 as I suggested earlier.

For example

a [at 3% damping] = {0.05 / 0.03}^0.2 x a [at 5% damping]