Water additions on a recipe now have their own unit dropdown, so a small addition no longer has to be converted into your account’s production volume unit - you can enter 5 gal rather than working out that it’s 0.1613 bbl.
Add or edit an Add volume of water action on a recipe (Production → View → Recipes) and choose the unit next to the quantity. You’ll be offered litres, hectolitres, and US gallons/beer barrels, or British imperial gallons/beer barrels if that’s the unit your account works in, with your usual production volume unit selected by default.
For more, see Recipes & brew sheets.