Mark,
You can use the FAQ baseline settings for a good starting point on your 450. The fork and shock on your bike are very similar to the units supplied on the 250/300 2 stroke, unfortunately they even used the same fork spring (.42, too light for you and your bike, consider a .44 or .46 depending on your requirements) and they increased the shock spring rate to a 54 nm from a 52. The stock spring combination gives it a "stinkbug" attitude and is not balanced front to rear.
The 258 mm is the set length (or, installed length) of the spring, the free length is 270 mm (give or take a couple of mm for manufacturing tolerance) the difference is 12 mm, which is the amount of pre-load applied to it. At this point, set your sag according to the FAQ, 105 to 108. This would be a good starting point, feel free to give me a call if you need.
Les
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