Hiya Metal wonder nine guy and thx for your thread. Suppose we can use some small amount of distinction in your question. The Kadet,a full framed .22lr pistol verses are you asking about one of the several Kadet adapter its avail to use as a conversion.
I've generally not seen in advertising of the factory models what the weight of the main/hammer spring is,nor found it listed in the owners manual, for the 75 series. Where have you found that its the standard weight of each?
If your question is coming from, I just bought a Kadet full framed gun and the original owner has never changed the main spring, do I need to change it to prevent light strikes? I would test drive the Kadet and if no light strikes occur,enjoy you gun.
Generally speaking if you research the Kadet/kadet kit area of out forum the trend is to suggest one needs a 17lb main spring to prevent light strikes,if you are aware you have a 20lb,that will work all the better.
If you meant to say Kadet kit, then I'd mostly agree with canthelpit,and recommend the 17lb over the 15lb main hammer spring. When I looked at kadet kits for sale, it can add to some misunderstandings as they seem to have included several recoil springs and designate a color to ammo,without mention of lbs of weight of the spring. i.e.. high velocity,normal,and low to colors of the spring. Just my humble opinion that pricing the kit another 5bucks,adding a main spring and small note in its baggie. "cz recommends this 17lb main/hammer spring if you actually want all you ammo to go BANG",and would have saved much grief to operators of the kits. Be well.