Spring requirement doesn't vary with shock manufacturer. All of the shocks made, OEM, or any aftermarket, all have to deal with the same suspension link geometry, bike weight, added load, etc. That's a red herring.
The spring rate will determine what range of the shock's travel the suspension will go through with a given amount of bump input, for a given amount of dead weight (that's your bike weight, you and any passenger load plus luggage) assuming perfect damping.
Now comes the big conundrum: How much shock travel do YOU want? That will depend on what your riding habits and your goals are, and not subject to some fancy formula that they have determined at XYZ racing shock seller company. And a lot of people don't quite understand what they want, which surely complicates the whole thing.
Truly race performance oriented riders will want to restrict their suspension articulation, because that also restricts the variation of steering angles and other badness, and will result in more predictable performance on a relatively smooth surface. When you get a race bike really dialed in, it is surely a thing of joy within the expected undulations of a race track. Unfortunately for most of us, our streets are nothing like race tracks.
In the extreme opposite situation, riders who ride off-road (or the roads they call paved around here) NEED far more compliance from their suspension to keep them from careening off-piste at even moderately aggressive speeds, and so you see, the target is a widely moving one.
The perfect spring rate can easily be determined if...
You know how fast, how rough the road, how much load, and how much you want the suspension to deflect.
See... it's easy. That's why everybody is doing it.
