... according to one Bob Burnham

"The best source on purchasing food during the Peninsula War is Major Alexander Dickson's Petty Cash Books. The books cover the years 1809-1811 and provide clues not only on the types of food available but the cost of the items. The following items are taken from the books and includes the cost in both local currency and in British shillings. [21] (The conversion rate is based on the 1811 exchange rate of one Portuguese Dollar equals 40 vintems, which equals 4 shillings 6 pence. [22] Note: One pound equals 20 shillings; 1 shilling equals 12 pence)"

https://www.napoleon-series.org/military/organization/c_mess.html

Best wishes
A