wet enough for metallic to flow even but not too wet where you are getting mottle/unevenness.
I see guys generalize every basecoat into one, and say never spray wet....medium wet only. Well if every based sprayed the same and was made exactly the same, you could say that. There are wet spraying bases and there are not.
You dont want to spray it wet in terms of the way you would clear, but it HAS to be wet enough for the metallic the flow out.
I use an iwata lph400 with 1.3 tip.......at around 20 lbs with sikkens base. With the wanda base i back the fluid out a tiny bit more and up the pressure a few lbs, as it is a thicker type base. doesn't spray as wet as sikkens either.