You are probably either replacing the coil on the shock you are already riding or you are upgrading from an air shock. Here is advice on both situations below:
Note that your Sprindex includes the spring, adjustment system assembled to it, and all Adapters to fit nearly all shocks. Note also that you should identify your shock's actual rated stroke and not go by the printing on your spring because oftentimes shocks come with springs that have more travel than the shock requires.
Consider which of the categories you fall into:
Note that sometimes the rate marked on your current coil shock may be up to 5% or more off of reality. The only way to know for sure is to have the rate tested. As a point of comparison, you can use a spring calculator such as this one from T F Tuned:
Spring calculators do not take into account your exact bike suspension, the way that you ride, body position, trails you ride, and so on. They can be a good guideline starting point.
Assuming you are buying a new shock, contact your bike brand for coil spring rate advice. Or, at the least, use a spring calculator such as this one from T F Tuned:
Then buy a Sprindex with that rate somewhere towards the middle of the Sprindex range.
Ideally you can borrow a friend's coil(s) in order determine how close that coil is to your ideal. If the SAG seems anywhere close to correct, go for a ride and note the ride quality and whether you are nearing full travel anywhere on your ride or bottoming out too easily. Then follow the above advice for a coil shock you've been using.
Determine what your shock's stroke is and then compare that to the available Sprindex springs. Choose the Sprindex spring that has at least the amount of stroke that your shock requires, and then measure whether that Sprindex overall length can fit between your shock's flanges. The Sprindex overall lengths are listed in the compatibility table. Note that most shocks can only achieve about 5mm less than their listed stroke because the bottom-out bumper can only be compressed so far, so in most cases it is okay if the Sprindex coil has up to about 3mm less stroke than your shock's stroke rating.COMPATIBILITY CHART