There's a guide for the values of item and resource stockpiles here:
http://forums.taleworlds.com/index.php/topic,324105.msg7650722.html#msg7650722
Simple and short:
Edit value 1 of the stockpile to your desired percentage of the default value. Ranges from 01x - which is 20% - to 12x - which is 1000%. 00x is the default value - so 100%. "x" represents any number (and a faction in this case), you should not write an actual x in the value field. You can find these percentages when you press F2 in the editor and scroll down a bit.
Often you can not see the direct result in the editor. You might want to restart your game (to reload the scene) or use an actual server to see the result