Like others have said the envelope is defined by the liklihood of success. However, air speed, pressure altitude and time gets into this. Remember you are probably getting closer to the cows or even worse, mountain tops. So don't squander your air speed and altitude by stretching out unlikely attempts at restarts.
I have seen some analysis (at lower altitude) where the only chance was to push the nose over and dive. Of course all engine restart envelopes are not the same if only because there are other trades to consider.