Mine was from schluter which is taken from BS, apart from the wood floor bit which was advised from another company who had a problem so more precaurionary than gospel.
I like you thought it was done by square meterage which i posted originally, but then spotted in the schluter notes it wasn't based on overall area.
I shall have a look at the latest BS tomorrow to double check, but if you were to do it based on length/width, it can't go wrong as the areas could be smaller.
Thanks for the bit about the perimetre. Another bit i can add to my ever growing book of notes