why convert to USA since most of the data in LFS is metric. So then your spring rate is equal to the mass*accel of gravity (kg*m/s^2) which gives Newtons, or killo-Newtons(N/1000). once you have that divide the answer by the travel on the suspension that you desire to get the spring constant required to carry the load over the travel of the suspension.
if this is any help.
Don