IMO, this is a good call and it is at least a better specification in that it's defined as a fixed center to center. An arbitrary "effective" chain-stay length would not be much use to say reproduce a specific geometry — frame building. Depending on where the axle is, the angles will correspondingly change — not by much, but they will change. The OP is just trying to fit a rack, so I can't imagine that such fine points will matter. But for argument's sake, I think the Vitus drawing above is a better spec. It seems to show the apex of the the d.o., but even if it is defining the apex as being the axle center (as loneviking16 says), when the axle is cradled at the extremity of the d.o.'s, it is a workable, fixed specification.

Perhaps the members over on the frame-building forum would really have the final say. This stuff must be second nature to them.

I like the Schwinn drawing. I am noticing that the "frame size" is measured as C-T rather than C-C — a real source of confusion