Ive been brewing 5 gallon batches for a while now and if Im making a lighter beer with a starting gravity of around 40, it will take about 10 lbs of grain. Ill heat 6 gallons of water to 185 deg F and set another 2 gallons aside at room temperature. Then Ill start off with 2 gallons of the hot water in my 10 gallon Gott cooler which is my mash tun.
Start adding grain and stirring with a spoon. When things get so thick that you can no longer get the grain on top wet, add some more hot water. Try to get as much grain in with as little water as possible.
When all the grain is in, check the temperature and add hot or cool water to reach desired temp. 153 deg F for sweeter beers or 147 for dryer beers.
Hints: Its easier to cool down the mash than it is to warm it up, especially if you can not apply direct heat because youre using a cooler for the mash. So aim high and be careful about adding cool water.
Once all the grain is in, do not over react to an initial high temp reading. It takes about 5 minutes of stirring for temps to even out the initial temp of the cooler and all the other variables will conspire to bring the temperature down. So wait and stir for 5 minutes and then make your adjustments using hot or cool water.
The water to grain ratio may be 2.5 to three gallons of water for 10 lbs of grain. For me, the ideal mash will have enough water to soak all the grains but not so much that they are swimming around freely. Remember, for conversion of the starches to work, the enzymes must be in physical contact with the starches, so you dont want to dilute things too much.
For rinsing the grains, I use the remaining 4 gallons of water and try to end up with 6 gallons of wort for my 5 gallon batch of beer. I louse about a gallon when initially transferring from the boiler to the primary fermenter.
For higher gravity beers, I will use more grain and rinse with more water. For a Bock beer, I may end up with 8 gallons of wort and then boil it for an hour to get it down to the 6 gallon mark before taking an OG reading.
I hope this will give you an idea of what the water to grain ratio should be, but its really more of an art than a science.