As you suggest, the spine thickness and blade width determine a sharpening angle that provides a “comfortable” shave. There is no, one, perfect geometry, but inclusive angles of 16-20 degrees are common.
As to the blade width. I don’t know what measurements came from razor manufacturers back in the day. The metric system has been around well over 200 years and has been in common use since then. Today I see vintage razors listed with blade width in both millimeters and eighths or sixteenths.
As you suggest, the spine thickness and blade width determine a sharpening angle that provides a “comfortable” shave. There is no, one, perfect geometry, but inclusive angles of 16-20 degrees are common.
As to the blade width. I don’t know what measurements came from razor manufacturers back in the day. The metric system has been around well over 200 years and has been in common use since then. Today I see vintage razors listed with blade width in both millimeters and eighths or sixteenths.