I have the same pot, so I know exactly what you're talking about.
Within reason, it doesn't matter much, since the water/grist ratio isn't really that critical. But you're right that if you go too thick on your mash you won't cover the grain.
I do a single batch sparge so I follow denny's advice of aiming for equal runnings from the mash and the sparge. So let's say I'm shooting for a pre-boil volume of 7 gallons, which means I'm shooting for 3.5 gallons of first runnings. Say my grain bill is 14.5 lbs, so I add 14.5*0.125=1.8 to account for grain absorption. Finally, I add the dead space of my kettle (i.e., how much water is left behind after I drain), which for me is about half a gallon. So I end up with 3.5+1.8+0.5=5.8 gallons of strike water. Then the sparge water calculation is easy -- just half the pre-boil volume, or 3.5 gallons.
Hope that helps!