Well, do the measurements of your design, find the force multiplier. As far as weight, you could guestimate, maybe 380 lb bike plus 170 lb rider gives 550 lb total. Then figure maybe a 30/70 weight distribution, 30% on front, 70% on rear. That gives 70% of 550 lbs is 385 lbs on rear wheel. Of course wheelies puts it all on the rear. Passengers add a little, too. So, figure a minimum of 385 lbs times your force multiplier to get starting point for your spring. Do the same for the other weights to determine final spring travel force. The difference between the two spring forces, and the travel desired of the spring, will give you a target spring rate (lbs/inch).
For example, if the spring must supply 550 lbs at normally seated ride height, and 800 lbs with passenger, not fully compressed, and the desired shock length change is, say, 2 inches, then the spring rate would be (800-550)/2, or about 125 lbs per inch. At that rate, the spring would need to be initially compressed about 4 1/2 inches to get the initial 550 lb force.
Again, this is just an example. Define your design geometry, measure, and see what you get. Sometimes, making side-view drawings, cutting out pieces, pinning parts to see the movements helps a lot.