Cisco has some default values for the max threshold and the mark probability denominator. It's proportional to the precedence and DSCP; the higher the precedence or DSCP value is, the less likely it will be dropped. In order to know the default values, jut enable random-detect under the interface then do show queuing interface to see the default values.
On the other hand, I would suggest you to watch the Advanced Technologies Class Bootcamp, Day 9 QoS part 4, Brian McGahan did an excellent job of explaining randon-detect.
HTH
Malick Ndiaye
"Even a stopped clock is right twice a day" 