Priority Queue - Bytes, Kilobits Etc

Just trying to understand this a little better.

In my lab I have 2 x routers - 1 x serial connection between them.
Each router has a LAN and I have set up a simple QoS policy and given
the Priority Queue 50K for host to host communication.

The serial connection has been configured at 128k.
The show queueing int serial 1/0 command gives me 48kbps.

A 1500 byte ping fails and the priority queue drops all my traffic.

I can rectify this no problem, however, I am trying to get clear in my
head how I calculate the correct Priority Queue / Bandwidth figure to
allow this ping to succeed without changing the MTU.

Secondly, how does the interface decide that 48k is available.

Regards

Darren

darrenfgreen@tiscali.co.uk