For the new Airbrush Developer, the mixing ratio is 1:1, consistent with the Creamy Oxydant. This means you should mix 1 drop of Liquid Dye with 1 drop of the Airbrush Developer.
Typically, for one treatment, you would need 5 drops of Liquid Dye and 5 drops of the Developer. Be sure to use the backflush technique to mix the two components in the cup. It’s also recommended to let the mixture sit for a few minutes before applying it to the brows, allowing the dye to oxidize properly.