ENDIAN Firewall - Yet another QoS question

Anyone know of any definitive information on how to configure QoS on EFW? It really doesn't seem to work the way you'd expect. For instance, I've created a new QOS device and set it to my main uplink. I set the max downstream to 1600 Kb/s and my upstream to 100Kb/s. I have my Uplink Main - High Priority class at the top of the list. I haven't defined any rules.

When I download something it looks like it is using approximately half of my downstream speed. Why is it doing this when I don't even have any rules defined? Should this be controlled by the rules?

Here is what I'm trying to accomplish. Maybe someone out there can explain to me how it can be done:

I would like to reserve 50% of my bandwidth for a specific service which uses a specific port (let's say 563). However, if there is no other traffic on the uplink originating from my LAN I would like it to use 100% of my downstream speed. If there is any other traffic I would like that traffic to be able to use the other 50% of my downstream bandwidth.

Seems pretty simple, doesn't it?