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The old parameterization (https://github.com/official-stockfish/Stockfish/pull/2225/files) has now become quite inaccurate. This updates the formula based on updated results with master. The formula is based on a fit of the Elo results for games played between master at various skill levels, and various versions of the Stash engine, which have been ranked at CCRL.
Skill 0 .. 19, now covers CCRL Blitz Elo from 1320 to 3190, approximately. Indeed, the Elo of stash in this analysis is only to within +- 100 Elo of CCRL, probably because it depends quite a bit on the opponent pool.
To obtain a skill level for a given Elo number, the above data is fit as a 3rd degree polynomial Skill(Elo). A quick test confirms the correspondence to the above table:
No functional change.