I do a lot of all grain batches around the 2.125 gallon size, using Mr. Beer fermenters and my 5 gallon mash tun.
I used a Mash Water Volume Calculator
to determine that the total volume of strike water and grains will take up 2.66 gallons of volume in the mash tun. (Enter 6.77 lbs. grain and 1.25 for mash thickness)
I calculate 6.77 pounds of grain with a 1.25 quart per pound mash thickness will 2.11 gallons of strike water. (6.77 * 1.25 = 8.46 quarts strike water)
I calculate 6.77 pounds of grain with a .13 pound grain absorption will absorb .88 gallon of strike water. (6.77 * .13 = 3.5 quarts water absorbed)
8.46 qts strike water
3.50 qts absorbed
11.96 qts. total strike water for mashing
I just heat up another pot of water, about a gallon or so, to 168F and fill my 12 quart boil pot up nearly to the top. But then I lauter an extra .5 gallon of wort into another small pot, bring it to a boil and add to my boil pot as needed to make up for evaporation and hop absorption.
Doing it this way allows me to do full wort boils and have enough wort to put in the fermenter without having to use filtered water to make up the losses. Not to say this is the best or only way of doing it but it works for my brewing equipment.