I am sure no whiz at this but if it were mine ....here is what I would consider...
Is this shock spring original? Meaning did this shock spring come on the bike new from the factory?
If the answer is yes then take a look at the parts manual. This manual should give you the part number and then within the description of this spring part number should be the spring rate....then well with KTM the parts manual will show you different options for springs to purchase. But I feel this would be an excellent starting point to start you on the path without spending any money. For KTM bikes, yes, I know this isn't a KTM, our bikes come set up to accommodate riders in the 170 pounds weight range. With regard to not mixing them up how about a different color /dab of paint on the spring to denote which is which? I can't overstate the value of regular service on your front and rear suspension.