I've found that around 70% of my connection's max upload speed is the sweet spot for keeping things speedy, but I only do that if I want something fast and there aren't many seeders. I typically download at 20% of my bandwidth and upload at 10% so when it's rocking I don't affect anyone else on my network. I don't have symmetrical up-down so my upload limit is a little above 10% of my download limit.
For Linux ISOs, of course.