OK, I actually don't know if the last line is true or not. I've not seen the method enough to know it, but if it's not true, then the method is A) not symmetrical. B) cannot be used to compute the age of anything else, and C) Cannot be used by the twin that stays home. This seem unituitively wrong to me, and I lay odds that if his paper is read in detail, that I got it right. It is a way of mapping every event in that light cone, and it is self-inconsistent if it cannot be applied to any other object except inertial ones. The method is actually quite simple and I don't think of it in the complicated terms of an imaginary twin at all. Where's the paper? I looked through ON THE CONVENTIONALITY OF SIMULTANEITY and didn't find it described. Maybe it's elsewhere. This is the same thing as saying the (square of the age of the other twin) plus the (square of the interval between this twin and the reference event) is zero. No need for an imaginary twin, or a bunch of them.