The design of ribs as it relates to plastic moldability is really quite simple.
THE DIVINE 66% RULE
The thickness of ribs should never exceed 66% of the nominal wall thickness.
If your ribs never exceed 50-66% of nominal wall thickness you will never have a problem with sink.
This demo illustrates the sink effects to be expected if you exceed the 66% rule.
Okay, rules are meant to be broken right? Sometimes you can get away with 66% to75% of nominal wall, but it is risky. Don't do it unless you absolutely have to. If you do, be certain that the area gets better than average plastic flow so that it can be packed out well without overpacking the rest of the part.