You have to calculate the difference of the current bone angle and the desired angle, then apply that difference by ent_bonerotate - or at least I'd do it this way.

Like

vec_for_bone(gvTemp, my, "bone14"); // get position of bone14
vec_for_bone(gvTemp2, my, "bone13"); // get position of bone13
vec_sub(gvTemp,gvTemp2);
vec_to_angle(gvTempAng,gvTemp);
ang_for_bone(tempAng,my,"bone13);
ent_bonereset(my,"bone13");
ent_bonerotate(my,"bone13",vector(0,ang(tempAng.tilt,gvTempAng.tilt),0));

Not tested though, but I guess that's the theory.