Thanks for your reply and for the Newberry reference, Cole.
The document has been translated and transcribed, and it is a letter from White to his superior, Governor Carondelet. Mad Dog planned to visit White to apologize for his (alleged) role in the killing of a PLC trader in Upper Creek country. The Creeks' war with the Chickasaws presented him with an opportunity to do so. I like your idea about a thumb as a unit of measurement. That makes sense.