Consider which of the categories you fall into:
Your current coil seems a little too stiff and you are not quite getting full travel. Choose a Sprindex with the highest end of the range at or just below your current coil. For example, if your current coil is 450, choose about a 400-450 Sprindex.
Your current coil seems a little too soft and you occasionally bottom out. Choose a Sprindex with the lowest end of the range at or just above your current coil. For example, if your current coil is 450, choose about a 450-500 Sprindex.
Your current coil is way too stiff. Choose a Sprindex with the highest end of the range around 25 to 50 below your current coil. For example, if your current coil is 450, choose about a 350-400.
Your current coil is way too soft. Choose a Sprindex with the lowest end of the range around 25 to 50 above your current coil. For example, if your current coil is 450, choose about a 500-550.
Your current coil seems to be very close to your ideal for most riding conditions. Choose a Sprindex with the your current coil rate somewhere towards the middle of the Sprindex range.
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.
Choose a Sprindex for upgrading from an air shock.
Assuming you are buying a new shock, 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.
However, if this isn't possible, then for most air shocks, multiply the air pressure (in psi) you have been using by 2.5 and that number is usually close to the coil rate that you will need for your coil shock. For example, if your air shock has 150 psi in it, then a coil shock would probably need to be around 150 x 2.5 = 375 lb/in. This conversion is not a perfect system and will not work for all shocks, but should be somewhat close if you have no other way to estimate your optimal coil rate. Then buy a Sprindex with that rate somewhere towards the middle of the Sprindex range.
If your shock is not listed in our compatibility table.
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.
