Nginx speed rate patch

Hello I need a small patch for Nginx. Right now I use limit_rate and limit_rate_after configs. I want that limit_rate_after to also be subject of a third config that will limit the rate. Basically I want 4M of data sent at 500KBps then the rest of the file at 90KBps. The bidder should be aware of Nginx code and know where to edit efficiently. I will ask you to tell me what file is to be edited for this to work, I know the Nginx source so please don't give me generic messages. Thank you!