We used to teach that you keep 20 miles away form TS cells / lines since hail can be projected out of tops for some miles. From the AIM, IIRC. Con, your anecdote (50 miles) makes chills go up my spine.
Yes, the standard is 20 miles, but I was in trail of a American Airlines 727 years ago and we were all diverting around the front of a line of thunderstorms going into the New York City area. We were at FL 240 and were about 50 miles in front of the line when the American flight encountered the hail and declared an emergency.
So since then I used at least 50 miles as my buffer distance when downwind from thunderstorms, when I could get it. I also remember that the jet stream that day was really unusually strong for that time of year.
I was flying a Jet Commander, so tells you just how long ago this was.