I don't remember where I originally learned this (other than "on the Internet") and it doesn't seem to be well-known, so I might as well post it here for others to learn: you can convert spring rates between lb/in and kg/mm with a single multiplication or division operation, and there's only one number you need to remember:
55.998
Multiply by it for metric-to-imperial, divide by it for imperial-to-metric. I've also seen the number 56.007 used, although that's splitting hairs for doing calculations on automotive springs.
