You don’t know anything unless you’ve measured it. A very wise tuner passed that statemet down to me. I learned a lot from him. Dan Kyle. Look him up.
People get pretty spooked about suspension. They get lost. They get confused. I used to. Then I bought a spring scale and everything made much more sense after that. If you arn’t measureing your springs you’ll never get it right.
Here’s a graph showing some actual rates for Fox 40 springs. Close to rated. 5% off roughly. Motorsport springs are usually within 1%. Fox rear springs are claimed to be withing 15%. That’s pretty bad. The term for what you are looking out for is suspension coil rate overlap. Hyperco describes that HERE