I calculated the spring constant for the Hs=80 rubbers, from the adding test (on 24) and removing test (on 27).
spring constant [N/mm] = [kg/mm] * 9.8 m/s2 (grav. acc.) / 3 (# of parallel rubbers)

The results were 469 N/mm (adding test) and 572 N/mm (removing test).