Yes I do that to. I just find that hardwall don't rule as flat as sand and cement. And floating the hardwall I find the float just glides over it.
What size darby? Surely not a standard 1.2, I've got a 1.8, 2.0 and a 2.4 for floating and normal 1.2 for external rendering.if high suction we 2 coat hardwall, first coat kills suction, lay on 2nd coat and rule in, we use a featheredge rule, a lot of plasterers use a darby.
if low suction, may put the room on before i start to rule, then check to make sure it has not slumped.
we leave the hardwall pull in, then with a light spray of water devil up with a diamond float.