Just reach back and feel for where your bones are when you are riding. I guess as long as you are truly comfortable, it doesn't mater. My indentations are well in from the rivets. A line drawn across the seat from the most forward rivets would go through my indentations. I have 6' 3" torso size/reach, and ride a 58" bike with a 130 stem, standard slightly offset seat post. I have a hard time believing you would be comfortable riding the rim.
My B17 has over 1000 miles on it and the indentation are easily visible where they are supposed to be. There is perhaps a little bit of stretch back to the rim, but not particularly evident, certainly not major indentations.
I remember reading somewhere that the brooks saddle does not have a lot of rail adjustment, and can be difficult to get all the way back. I was wondering whether to make my seat post more raked in order to give the saddle a more nutral starting position. But in reality, I don't think the position on a 73 degree seat tube is all that bad.