A good conservative rule of thumb that I use is never fly downwind of a cell if at all possible. On occasions where flying upwind of the cell isn't possible, I use 1 nm per knot of wind coming off the cell for minimum distance for avoidance. For example, if winds coming off a cell are showing 85 knots, I'll pass at least 85 nm from the cell, but never less than 25 when above FL250. Also, I read that the flight was running about 90 minutes behind schedule. I wonder how much, if any, the pressure to make up time and get back on schedule played in this event. Here's an excellent tool I use in my flight planning: Storm Prediction Center Convective Outlooks

I hope some of this is helpful.