The source data used for this computation comes from the FAO Global Soil Organic Carbon Map v1.5 (GSOC).
Please read about their methodology in the technical report, it is fascinating! It uses World Harmonized Soil Database, SoilGrids, and many other sources from around the globe, for a total of over 1 million real soil samples. That said, an actual assessment with your soil samples will give a more accurate estimate.
This is one of the few things that the fantastic FAO Glosis tool doesn't do, and the moment they do I will take this project down.
The tool was created by Rikki Schulte for fun and for the earth, using GeoBlaze, powered by GeoTIFF.js. Check out the source on github
CC BY-NC 3.0 © Rikki Schulte