Just go by the air cell growth. You can’t really go wrong that way. And if you’ve had issues in the past then experimenting is a good idea. There’s so many variables for each hatch, so find what works for you! :)
I don’t know about Brahma eggs but I do breed silkies and hatching at a consistent 30% might be to low for them. Each hatch is different and you really need to go by how the air cells are growing and adjust humidity accordingly. But with Silkie eggs I am usually around 40-45% humidity, very...