Water is cheap. Heat up plenty of it. An extra couple of gallons is not going to run up your utility bills. You only need an accurate temperature for you initial dough in. Instead of going for an exact volume and then dumping the whole thing into your mash tun, use a graduated pitcher or bucket to measure out the water you need after it is hot. This eliminates having to lift a hot kettle. Also you have a head start on heating the water you need for the sparge. Just add more water after you get the mash going. As others have said, any hot water left can be used for cleaning. You will need that water to clean the mash tun.
Wayne
P.S. Also check out my post here:
viewtopic.php?f=4&t=29058