there is no point suggesting it was anything other than an inspired guess.
The reason I am responding to your post is not so much to answer you as to ensure the media are clear that your post is based on a lack of understanding. Please accept my apologies for that term because it is not meant to be insulting.
On the subject of inspired guesses versus the evaluation of the aerodynamic factors involved as mentioned by the above poster.
All Pilots with any experience are well aware of the drag factors evident in the use of Flap 30 or 25 (40 to 30 on 737, or whatever for a given Boeing model). They do not need to get a calculator out to work out the advantage the reduced drag will provide - they know it will will work to their advantage.
To suggest that "he guessed" is incorrect - he didn't guess, he applied his significant expertise and experience as a Pilot and Commander to ensure the best outcome under difficult circumstances.
It has been said before but I think it is worth repeating here. Such scenarios are not trained for, double engine failures yes, but at altitude. Double engine failure when fully configured on short final is the stuff of nightmares - it may seem as if this crew pulled a rabbit out of the hat, but no this crew acted correctly as a result of their training and experience.
Regards
Exeng