We don’t know yet, like you said it is very hard to choose the right parameters. We have to make simulations to understand the impacts of each parameter in order to choose them.
Each individual has the power to add a distance constraint to any other individual through the distance rule. Indeed, by certifying someone, you create a potential path from you, to this someone, to newcomers. By this act, you are becoming a control point of the WoT. Absolutely all newcomers will have to have a path from you to them at the moment the join/renew (each member has to renew, every month for example).
This rule, however, applies from you only if you issued enough certifications (for example 3 in MetaBrouzouf) so this rule does not apply for people who do not certify anyone (this would avoid paths to exist! and no newcomer would be able to join ever).
By this act of certifying, you create a tension in the WoT. The WoT cannot extend much more than 3 individual from you (in MetaBrouzouf). So when a sybil will have to renew, your possible paths may lead it to be actually banned.
Note how I subtly always avoid judgement. 
Because condemning someone to be banned is an extremely severe human judgement, with very deep consequences, relaying the judged “thing” to a non-human. We should be very careful on this point, because since we accepted the RTM we also accepted the non-nuisance principle (also callable non-violence principle).
That’s notably why I do not want of “active” ban with anti-certifications. Some people may want to actively ban someone for wrong reasons. I rather propose to use the pacific way.