Carbon silicate is the best board out there but hard to come by. You dab them 1 of 2 ways, either screed the wall as a whole so as to try not to leave any voids when pushing the Panel back, this is hetas preferred way, to me though there's always going to be a void somewhere with nowhere for the air to escape as its heated up.
All I do is dot and dab using roughly twice the amount you would do on a normal wall, making sure to leave a couple of continuous gaps from bottom to top to let the hot air escape past the top of the board into the chimney.