If you are talking about JSA -as opposed to JSB- then it is not so much how much you will earn from your new self-employed 20 hr thing-it is what you are worth overall.
They will means-test you. that involves how much money do you have in the bank, does your spouse earn anything ( I think that includes live-in partner) and a lot of toher tricky questions.-living free at home? In some cases they ask what car do you own? any nice holidays recently ?
I'm exaggerating a bit - but be very very careful when applying for JSA.