With the following config...
set ip precedence 4
priority percent 50
set ip precedence 3
bandwidth percent 30
shape average 512000 5120
description MPLS 2MB connection
ip address 192.168.x.214 255.255.255.252
service-policy output SHAPING
...would the priority percent use 50% of the shaped rate or 50% of the interface's bandwidth command for it's percentage calculation?
To add some explanation to this:
With your configuration the queue system is built up the following way:
Interface Traffic -> LLQ -> Shaping Queue -> Transmit Queue -> Outgoing traffic
So if you configure the 'priority percent 50' command, the LLQ will allow 1000 kbps for the priority traffic.
But the shaping queue is configured for 512kbps only, so 1000 kbps priority traffic could overwhelm the shaping queue, leaving no space for other traffic.