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.
May I respectfully suggest that had he been at the correct Ref speed with landing flap set, and then retracted one stage, the first thing the aircraft would have done is sink. As far as we are all aware, he left the A/P engaged, so presumably without thrust he did not increase speed - we just don't know that part yet, which is another reason that suggesting Diagnosis, Option consideration, Decision, Allocation and Review was unlikely to have been carried out. Further, unless you have heard the CVR, it is not known who suggested / requested a flap retraction, or what speed/height/range formula was used. After the sink, preferably if the speed had been increased to optimum, (perhaps by a shallow dive), then the glide would have been extended. My point is that neither he nor anyone else could have accurately known the interface between the extent of the sink and the subsequent glide with a slightly higher IAS. Hence he guessed. Made an inspired estimate perhaps?
In smaller aircraft, in the days when I worked for HMG, the practising for forced landings covered many many parameters, on approach and on departure. One thing I recall over a number of different aircraft types was the stomach clenching nature of that sink when one retracted a stage of flap at a critical speed and with the engine simulated failed.
.......and yes, I fly Boeings too!