Try 10%-ish.
Add an ounce of softener to 9 ounces of regular for 10 ounces total. Add a little stabilizer when you add softener or hardener because softeners/hardeners do not come with stabilizer.
For a 16-ounce total mix, I'd add 1.5 ounces softener to 14.5 ounces regular.
For 20 ounces total, I'd add 2 ounces softener to 18 ounces regular.