Hi Dave,
Based on my collection of reference photos 8 to 10 lines per inch would probably be right if you are looking to duplicate the checkering that shortly preceded the finer more modern English style rather that the really wide cross hatching that was done in the 1770s-1780s. Unfortunately, I have never had the chance to handle a pistol from that era, only ones from the 19th century that had finer checkering. My guess based on the depth of the lines is that they were cut with a checking tool with coarse teeth much more like a saw than a file. Sorry that I cannot be of more help. Jerry H or perhaps Taylor might be of more help.
dave