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You don't. The boil volume is back calculated from the batch size (volume to fermenter) through the process using the process specs you indicate in your equipment profile. You can follow the volumes in the 'volume' tab in your recipe if you have a question on how the program is coming up with the numbers. Given what you describe, I would suspect that you are topping off the boil kettle or the fermenter with water and that cuts down on the volume which is remaining to participate in the mash and sparge.
If you want more specific help, export a recipe with your equipment profile as a .bsmx file and post it here. That way we can see how you have your system set up and offer some specific advice. you can export a recipe (or any other profile) by right clicking on the recipe and selecting 'export selected'.
All right, your mash profile has a mash in and a mash out steps with a batch sparge. The volumes are identified for the initial infusion and for the mash out in the profile. When you apply these profiles to your recipe, the program will calculate your sparge water to make up the required volume to get to your pre-boil target. You should see this in the mash steps of your recipe.
As I don't have the mobile version, I cannot point you where to look. Hopefully someone who does can chime in to give you specifics on where to look to identify the sparge volume in the recipe.