I started off with a 10mm trowel, but the walls were not as straight as should be also there was a difference in thickness from the backer board to plasterboard areas and the partition wall not 100% 90° to the wall. So have dot n dabbed a lot,have used a good amount of dot n dab, not just a little on each corner, have used as much as would have been trowel on,they are solid not gonna come off.
The big walls needed to be straight as having fitted units along 2 of the walls both base and wall.