Allowed and excluded regions for the formation of an sdB+WD binary through stable mass transfer similar to PTF1 J2238+7430. The central green-shaded region indicates combinations of initial donor mass (M_1, init) and orbital period (P_init) that lead to sdB formation consistent with observations. The hatched and colored regions correspond to physically forbidden configurations: violet (left) shows donors too low in mass (no He ignition); red (right) shows donors too massive (oversized core or Roche-lobe overflow in the subgiant phase); blue (bottom) shows systems at risk of common envelope or merger events; and orange (top) shows donors with thick hydrogen shells preventing sdB formation. The black star marks PTF1 J2238+7430 parameters, while black squares mark model systems that produced potential double-detonation supernova progenitors with slightly different masses from the observed system.