It being NOS could be a case Against milling it down. I would be more inclined to do that to a used post rather than a NOS one. I would think the value of the post in its original condition and size would be higher than as a milled down unit. Of course, if it is going to stay on that bike forever and ever, then no big deal, but the post is still worth more in its original size than milled down, even if it is no longer NOS. But in the end, it is up to you what to do with it, and milling off .4mm should not damage integrity, they are pretty stout posts. As for milling only to the limit line (which would make sense aesthetically), that pretty much leaves you with only that option, fitting it to the limit line. It would go in no further, if, for instance, you wanted to have the post farther in (lower seat) for your particular needs and application,,,You would be stuck with the amount of seatpost you milled it to.