Derailled

Where does one measure from etc? For a shock that is.

Or can it be calculated this way.
Bike one 5.2 travel 7.625x2.0 stroke.
Bike two 4.5 travel 6.5x1.5 stroke.

Basically I've got this spare shock (and frame) and going through all the relevant Specialized comparisons. The only difference to the 04 SX (my bike) is the linkage, which slackens, lowers and shortens the travel. So it must be just the linkage that's difference.

Either way the shock bolts on, but the bushings are a fraction too tight so I can't go for a ride. Another inch on the back would be nice, it feels like I'm riding a hardtail most of the time on this anyway!

Shock length is irrelevant to leverage ratio calculation, you are looking for shock travel vs. wheel travel, I worked out my Demo 7 has 190mm of wheel travel/63mm of shock travel = pretty much 3:1. This was useful in selecting the right shim for the Rock Shox Vivid I bought, turned out I needed the C tune.

But now you've posted all this other info the plot thickens beyond my knowledge really. You might be able to bang in a longer stroke shock that is the same i2i, but you might hit the tyre on the frame or crunch something somewhere, if it also doesn't feel like pooze and use it's travel funny and the geometry go all whack... But then if you have different linkages, this might help, guess you just need to try it all out or get a smart engineer to mock it all up.

That bike was designed to be pretty stiff anyways eh? Like a short travel jump bike for 4X and stuff?

Ah cool, so
2.6:1 and 3:1

Well you're on the wrong forum me thinks...

Another thought, I was giving the shock etc a bit of a clean this morning, the i2i on it is 175mm, but when you pull the lower mounting bolt off, the rear end drops another 80mm? Is this clearance for simply removing the shock? Seems kind of odd to have this unused negative travel (260mm's worth). Obviously it's not sag, as the shock is topped out when it's mounted in there.

Do other bikes do this? I don't recall on my Big Hit having such a big difference from the rear suspension being topped out and with the shock in place.

Depends on the bike, on most of them it's cables which restrict how far down the suspension will drop without the shock. Then it's often links hitting frame/derailleur or getting close to turning inside out. At this point the geometry of the links have changed enough that your average leverage ratio is quite different to how much each mm of shock length/travel would be acheiving at the axle.
Of course you've just found there's nothing stopping you from bolting a much bigger shock in and seeing how it goes. 175mm is a strange length but not that far from 190mm.

Hmm, supposed to 6.5 inches, or 162.5 mm. I just re-measured it's pretty close to 165mm.