Since english isn't my first language, I'd have a hard time correctly explaining the motion ratio on a forum so I'll let wiki do it for me. It's not totally complete, but it's not so bad.
http://en.wikipedia.org/wiki/Motion_ratio
@MattSCSS, where thinking exactly the same. When I said the best was to try and be as close to the wheel as possible, I meant when you can design a custom shock with desired damping and stroke. When I refer to "as possible", I mean you have to find the happy medium.
As for using shocks from another vehicule, it CAN work, but it won't be good unless you do a bit of math and/or trial and error.