Okay, so, my first source was “The Roman Empire and the Indian Ocean”, and I was continuing my continual reading of the book while waiting for a mushroom burger (delicious, btw, as were the fresh-made potato chips that came on the side). Took a minute to locate and re-read the info about the pepper cargo, and noted above the actual figure.
Then I poked around in one of the links, turned out to be a PDF of a scholarly paper about the cargo. The figure works out to 544 tons, sez here:
[snip] Some 20,500 talents of 95 Roman pounds each correspond to more than 625 tons, 544 of which (87%) was pepper*. Even if the weight of the 80 containers of Gangetic nard was relatively modest, it is clear from this reappraisal that the weight of the rest of the cargo would be enough to qualify the Hermapollon as a ‘very big’ ship in the eyes of the traders of the Graeco-Roman world. [/snip]
Playing Sudoku on the Verso of the ‘Muziris Papyrus’
https://www.ancientportsantiques.com/wp-content/uploads/Documents/PLACES/IndOc-Gulf/MuzirisPapyrus-Romanis2012.pdf
I’ve got that book!