Instead of pouring it over your grains, I would keep your grains in the sack and dunk them in 170 degree water for 10-15 minutes. Make sure you stir the grain (still in the bag) at the start. Then lift them out and combine the sparge water with your wort. Better conversion this way than simply pouring them over. 1-1.5 gallons should suffice it you are using less than 5 lbs of grain.