I will echo Guardians statement. 17# seems to be as low as you can go without modding the firing pin on the kadet. You should get 100% ignition with a 17# main. I say should, because I have seen CZ frames and top ends vary in regards to dimensions.
For reference, Full size STANDARD 75s have a 20# main, compact have a 19# main.
Wolff sells reduced power mainsprings for 75 series pistols relatively inexpensively. Shipping is more expensive than 2 springs.
Let us know how you make out!!!
Rich