depending on your connection theres a good chance that you dont even need to limit the download, but instead need to limit the upload. When you download something your computer needs to send a tiny bit of info back saying "hey I got that piece, go ahead with the next chunk" or something like that. The problem is if your upload is maxed out this info gets put in line and your download speed gets adversely effected. If you set your upload to unlimited and have enough things running to max it out, take that number and then limit the upload to 70-80%, doesnt have to be exact, we are just trying to allow for a bit of a buffer zone.