first you need to set up your profile with the correct pot size or mash tun and all the losses, what I do is just use the quick water requirements from the tools menu which pulls from your profile and the recipe
I was looking under "Brew Session" / "Mash Water Management"/ "Mash Calculator" and I hope I got it correct that way.
Mash Water Management
Strike Heat 7.0 L to 77.5 for a mash thickness of 2.50 L/kg. 11:59 pm 68
Infuse Add 14.2 L at 77.4 to change temp from 68 to 74. 12:00 am 74
Mash Guidelines
7 L Infusion 68 C 60 min
14.2 L Infusion 74 C 15 min
Starting Mash Thickness: 2.5 L/kg
I´m use to Brewtarget that have a button that calculated this for you