When you set up a field job you can enter multiple inputs. using this feature you can work out the amount of water needed for a spray mix.
In this example you can make one from an example spray (ml) and one for water (L) water as an input.
To set this up first set up an input called Water.
Tap "Inputs" and select "+ Create new input"
Set an input name "Water".
Select a unit (L) and give the input a type, in this case Spray or Other.
Now when you set up the job you can add the amount of water / ha as an additional input. Read how you can add an input to a job here.
If you are conducting the work over multiple fields you can produce a spray report.