For full volume, or no sparge, brewing, you don't worry about the mash thickness, and it isn't used for recipes where BIAB is the brew process.
In the recipe editor, you'll need to enter the full mash water needed, which can be viewed from the quick water requirements either in recipe tools, or in the menu in the top left.
Alternatively you can use the "add quick water requirements" button to add the estimated total water automatically.
Set up the recipe using your ingredients, make sure
the correct equipment profile (in the MORE section) is selected,
the brew method is Biab,
and the preboil, and postboil volumes are accurate.
Then save the recipe, and reload the page (press F5, or view recipe and edit recipe again). There's a bug where the quick water requirements aren't updated with some recipe changes.
Then click the add quick water requirements from the menu in the mash guidelines. It will automatically add a strike mash step equal to the calculated total mash water.