I'll take the pot off the burner until the first runnings are in the pot, then I'll put it back on the heat. I never leave the pot on the heat when empty. Accidentally made the mistake once and I smelled like my house was on fire.
For my sparge temp, I'll ususally bring the water up to between 182-184 to achieve the 168 range when I have 8-10 pounds of grain, and I'll bring it up to about 186-188 when I'm using more grain. This is assuming I'm mashing in the low to mid 150's and ssuming my mash ratio is around 1.25-1.33 qts/lb. Not a very scientific approach to doing it, but seems to get me in the 166-169 range everytime.
On another note, my last batch I double crushed the grain through my mill and my efficiency improved nicely ... around 10%ish. Another thing, I read that when you're grinding the grain with a power-drill, not to go full throttle. I guess it doesn't do quite as good of a job crushing the grain, so you should just go around half speed ... not sure why this is off the top of my head; has anyone ever heard this?