Turn a Best Sellers Rank into the sales it implies.
Estimate is the right word for a book you cannot see inside. For your own titles there is nothing to estimate: the real figures already exist, in your KDP reports and your Amazon Ads account.
TrueRoyalties reads both and shows the net profit of each book — plus its actual Best Sellers Rank on every store you sell in, with its history, instead of a number worked back from a curve.