Free length is not so important as there's other factors involved in the spring rate. This will give you an idea, but remember that this only works for constant spring rates:

http://www.iroczone.com/calcs/chassis.htm

This calculator isn't perfect, but it will give you a rough comparison from spring to spring, assuming thematerials are the same.

There's other calculators but you need to know the modulus of rigidity of the material, so you need to know the exact material to start with.