thats good, fill up your mash tun with water and let it drain in the same way you would with a mash.. then measure the water that is left. My 10 gallon igloo holds on to about a half gallon.
I did this too in Beersmith but lately I just tilt my cooler to get it all out. No sense in wasting half a gallon just because it is at the bottom. Now I leave Beersmith at 0 for lauter tun dead space.