No one knows for sure how your score is calculated. Most prep companies try to develop their own algorithm, as similar as can be to the official one.
If by the score calculator you mean that 800score-related web page, I've also tested some of my percentile scores and found it to be slightly inaccurate. For my q50, v47 the calculator returned 760, while I got 770 on the real thing.

If I were you, I'd stick to the scoring of GMATprep. It's the most accurate thing out there. Besides, it contains real retired questions, so it's closest to the real test. In my experience, the questions in GMATprep were very similar to those in the GMAT, although I did notice one too many word problems on D-Day.

Here you go.

https://www.beatthegmat.com/calculate-score-t36037.html

Hope this helps.