My experience (replacing stock springs with Race Techs on my Trophy) is that if you want the ride height to remain basically the same you should take the original springs with the original spacers and lay them next to the new springs then add the amount of spacer necessary to make the two sets equal in length.
---- = spacer
ZZZZZ = spring
Original
|-----ZZZZZZZZZZZZZZZZZZZZZZZ|
New Setup
|_____________ZZZZZZZZZZZZZZZ|
_________ = amount of spacer to be added
If the final length of the new springs and spacers is much longer than the original all you will be doing is compressing (pre-loading) the spring too much and loosing suspension travel. The amount of suspension travel you have is equal to the space between the spring coils. Once that space has been used up, the spring is totally compressed and your out of travel. So on a single rate spring you don't want to have more than a few centimeters of preload, after that your just using up travel that you'd rather have for dealing with bumps and such.
Hope this makes sense,
Keith
