"If you are using a picnic cooler for your mash tun, multi-rest mashes are a bit trickier. You need to start out with a stiff mash (e.g. .75-1 quarts per pound of grain), to leave yourself enough room in the tun for the additional water. Usually only 2 temperature rests are possible with this method because the amount of heat necessary to change the temperature of the mash increases with each addition. Reaching a third rest is possible if the change in temperature is only a few degrees. For example, raising the mash temperature for 8 lbs. of grain from 150°F to 158°F at a mash ratio of 2 quarts per pound would require approximately 2.7 quarts of boiling water."
http://www.howtobrew.com/section3/chapter16-2.html
http://www.howtobrew.com/section3/chapter16-3.html
