Note that Fontenot's equation is a function ONLY of the two velocities v_1 and v_2. In particular, it is NOT a function of the separation of the twins at the turnaround. That is a bit surprising, since we know that the EFFECT of a given velocity change on the difference in the ages of the twins at the reunion depends LINEARLY on their separation at the turnaround. But Fontenot's simultaneity method gets the correct result at the reunion, because the duration of the center section of the age correspondence diagram (ACD) DOES depend linearly on their separation at the turnaround. I.e., the slope of the center section doesn't depend on the separation at the turnaround, but the width of the center section DOES. For details, see https://sites.google.com/site/cadoequation/cado-reference-frame .