It's probably a mistake to try to be too systematic about units of measure prior to 1795. For Moxon, as the Wikipedia article indicates, these would be English feet, i.e., more or less the same as 12 inches American, but measures were less standardized in general before the Industrial Revolution, so it should be thought of as roughly 30 or 31 centimeters, give or take, not as an exact unit. Each typefounder or printer would have had his own measuring tools that might or might not give the same results as others'.