Ian, I'm not sure I understand your question. If you are talking rough-in distance, that's finished wall to the center of the drain, the actual distance needed will depend on the toilet you use. I assume you are going to us an AS toilet, so you will need to see the spec sheet and find out what the model you are going to install really requires. I think AS is similar to other brands, including Toto, in that the actual space needed is usually somewhat less than the rough-in size listed for the toilet. Of course, a toilet listed as a 12" RI will always fit a 12" space, but there will be some space between the toilet and wall.
On the other hand, if you are talking about side clearance, then it's 15" center of toilet to wall.