From that spring calculator site "The spring rate is the amount of force required to compress the spring one inch, and is measured in pounds."
Your current spring is rated at 550 pounds according to the label
Optional - to test it, put the spring somewhere you can measure the compression accurately, and load it with 55 pounds of weight. It should compress 1/10 of an inch and then rebound to the original length. Likewise it should extend 1/10 of an inch if you hang 55 pounds off it.
As long as you're not exceeding its elastic limits then the spring will rebound to its empty size (so says Hooke's Law) Otherwise it will permanently deform, but that will take a lot of weight.
Based on the numbers in your question the answers from the calculator are:
Single Pivot / DW-Link
CCDB/5th(CVT)/Man(SPV) 764 lbs
Fox/RS/Maz (shims) 859 lbs
This is a single pivot suspension:
4 Bar / VPP
CCDB/5th(CVT)/Man(SPV) 840 lbs
Fox/RS/Maz (shims) 935 lbs
This is a pivoiting suspension:
The other titles are brands
- CCDB is probably Cane Creek
- 5th CVT is probably Progressive Suspension Fifth Element
- SPV is Manitou air shock
- Fox is Fox Suspension
- RS is RS Suspension
- Maz escapes me sorry
- Shims is one way of adjusting the spring, by adding more washer-like obstacles to compress the spring sooner.
Given your bike is slightly non-standard, at the end of the day you're going to have to take a guess. If your bike has a see-saw pivot in the suspension, go higher or if its a straight squash then go lighter spring rate.
You could also consider the normal load you exert on the saddle when it bottoms out. Then imagine half that weight, and extrapolate from there.