The maximum rakeback percentage on Microgaming is 30%.
However, some rooms on the network offer bonuses in other forms. For example, StanJames has a system where users raking over $200 week, get extra value in tournament $. Although, not as good as the cold, hard, green stuff, these certainly still have value.
On top of this, affiliates run rake races, can offer customized reload bonuses, offer coaching and lots of other types of value.
Unfortunately, it's just not all that easy to calculate and contingent on how much you rake too.
I would recommend posting a Rakeback Request
to find out what kind of deals are out there for you at Microgaming.
Hope this helps & good luck!