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do you know how much you'll need in the kettle for your pre-boil volume? work backwards from there. lets say its 7 gallons for a 5.5 gallon batch. and lets also say you've got 10 lbs of grain. if you do a single infusion at 1.25 qt/lb you're adding 12.5 qts (3gallons, 1 pint). the grain will absorb about .1 gallons of mash water per pound, so you'll lose a gallon to the grain. if you don't mash out with another infusion to bring your grain bed temps up to sparge temp, and you have zero dead space in your mashtun (unlikely) then your first runnings will be 8.5qts. you need 7 gallons (28qt) in that kettle, so you've got 19.5 qts to go. i do a mash out and double sparge, so i usually divide what's left in two, but give a little extra to the first sparge, because that wort will have more sugar in it. if your MLT (i'll presume you're using a cooler) can hold the whole 19.5 qts put it in, stir, and drain. you've got the 7 gallons needed. if you'd like better efficiency i'd recommend a double sparge, so maybe use 11qts for the first, and 8.5 for the second. don't worry about absorption, the grains all soaked up what it can. you do need to worry about dead space. either ignore it or tip your cooler. ignoring it drops you efficiency because you're leaving sugar behind. tipping your cooler risks stirring up some grain that will end up in your boil, possibly causing tannin extraction or clarity issues. use beersmith or one of the online calcs to figure temp. expect to not hit any of your temps. keep lots of notes. good luck!