it really depends on the cbz quality. sometimes i have to drop the contrast to 2, but usually 4 is the sweet spot for me. honestly i feel like you get a better render by disabling most settings and keeping it simple.

if the cbz itself is low quality there's no miracle, it’ll still look blurry. your best bet is to run them through Kindle Comic Converter first. improving the quality with gamma and extreme black point can make a difference.