See the first post and the subsequent discussion, I think that study data is the objective way.
Where going from Altitude I multiply by the Bassett number, a starting point going the other way would be divide. I’ve only gone from seal level to altitude, not the other way, I found that I personally was a closer to the acclimatized number after a week or so. Coming back to Sea Level and a little rest, I can say I felt great and immediately crushed a handful of PR’s.