Sambo - check out this site for online calculators.
http://www.swayaway.com/TechRoom.php
Due to the different suspension geometries, it is difficult to compare spring rates between different manufacturers. eg. think about the different force you would apply to the spring if it was mounted close to the hub, as opposed to half way along the arm - all these things need to be taken into account. i would see what fellow Harrison guys have used, and Im sure harrison has suggested rates?
Cheers,
