For traditional QoS, the first step is to find out what your available bandwidth is - actually, not ISP advertised. The speed test(s) of your choice will point you there. It's worth noting that if your speed varies, such as slower during peak times, you get better results using the slowest top speeds you reliably get.
http://www.dslreports.com/speedtest
http://speedof.me/
The second step depends on the first. Through
extensive testing, poking, and prodding I have arrived at 87.5% of max as the sweetest of sweet spots. I set the upload and download caps at that percentage.
Start there and if you have more questions/issues, feel free to ask.