MIMF

Board still hasn't flattened. Company suggested spraying the underside of the cup with some water. Tried twice but it didn't do anything.
Any other suggestions?

I realize my questions depend on the wood but in general what thickness do you normally aim for when preparing your top/back?
What would normally be considered to thin?

Back is for an OM guitar. It is quartersawn walnut.
Came cupped in the mail on Friday from A&M WoodInc.
My shop is at 40-45% humidity.

Just bought a new back and when it came it has like a 1/4" cup in the middle of the board.
Should I send it back or try to straighten it? If straighten it whats the best way to do it?

thanks