I use iBrewMaster and I use a 10 gallon HD round beverage cooler as my MLT.
The AG equipment profile settings are to help you dial in your particular system - grain absorption loss and mash tun volume loss are ways in which the liquid you put into the MLT is not the same as the amount of liquid you get out (i.e., water absorbed by the grain during mashing; the volume of liquid that will reside in the MLT after you drain/sparge - typically the liquid that sits in the area below the ball value and that you cannot drain easily, necessitating it to remain in the MLT as opposed to being drained into the BK).
For example, I know with my system I need to put in about 6.5 gallons into the BK to end up with about 5.5 gallons of post-boil wort, and ultimately right about 5 gallons of fermented beer. The liquid loss is due to (among other factors) grain absorption, MLT dead space, BK dead space, trub loss, etc.
Experimentation and good measurements (volume, gravity, etc.) at all stages of beer marking is the best way I know of to dial in your particular system. If you are too short on batch 1, add more liquid for batch 2. Repeat until you have it set. Then you can back into your particular profile on iBrewMaster.