How to calculate encumbered mount speed?

[Nagual]

How do you determine the speed of an encumbered mount. I have found the multipliers for determining mount's CC. But I'm lost for the next step after calculating encumbrance.

 

[frankthedm]

Carry more than the light load and; 60' drops to 40', 50' drops to 35', 40' drops to 30'.
http://www.d20srd.org/srd/carryingCapacity.htm

Also note

A creature with a fly speed can move through the air at the indicated speed if carrying no more than a light load. (Note that medium armor does not necessarily constitute a medium load.)

Flying mounts can't fly in medium or heavy barding.