Yesterday I built a 5 gallon batch. In my 5 gallon cylindrical tun I added 3.5 gallon of water at 176F and 12 pounds of grain at room temperature. My temperature after doughing in and then waiting 10 minutes ( I always wait ten minutes to allow the masses to reach steady state) was 156F. I shoot for 155F+. I do not like 155F minus.
Then two - 2 gallon sparges at 185F each. This brings my grain bed to 168F.
I have arrived at these temps and volumes with experience and keeping notes.
As the slogan goes - "your mileage may vary" - Good Luck, Mike