I am having a block wall dot and dabbed with 12.7 mm plasterboard then skim finish. There is a step in the wall and I would like to put battens on the recessed bit so when it is skimmed the wall is flush all the way along. I assume the battens have to sit proud of the blocks as the thickness of dot and dab and is greater than just nailing the board to the battens. I would like to know how much the battens should sit proud in order for the board to sit flush on both sides?