You have approximately 17 pounds of grain there. I would expect you to lose approximately 2 gallons to grain absorbtion. The other gallon you could be losing to mash tun design... Seems a little high, but not necessarily out of the realm of possibilities. Now you shouldn't lose anything during the sparge because the grain would be fully hydrated. EDIT: Also, I'm making assumptions here because you appear to be confusing the terms "mash" and "sparge." I.e., you state: "60 min sparge at 154..."