Spring rate math

[karnage]

I found this but dont know if its right.Says 1kg mm is equal to 56 lbs an inch.So if the spring is rated at 13.1 kg mm that is 730lbs an inch.Is that right??Trac says for a 6 over arm at 250 lbs a 16.1 kg mm spring is needed.Thats 901 lbs an inch.So I went with a 900lb spring.