Affiliate plugin and membership - Setup help required

I have membership and the affiliate plugin installed on a new 'vanilla' WP site setup. (no MU no BP)

I do not understand how the affiliate works - I may be missing the point - should I be able to set the percentage of payment somewhere?

I can see the Affiliate System Admin in the dashboard but there seems to be frighteningly few variables to set.

can you help and advise please?