Ok I am no stranger to files and filing, for years I have extensively used files of differing types and currently I have on hand 62 files if I count the files in my riffler sets. However there is not a single checkering file in the bunch as I have always before just used three-corner files for that purpose, even using glasses and/or magnifiers the fact is my eyesight is not what it once was nor am I near as steady, :( So now I am about to buy some Grobet checkering files and plan to buy a 20 LPI and a 40 LPI but I am not at all sure about the coarseness rating for either? I find them in no. 00 to no. 4 and while I am well familiar with these ratings I just don't know how to best choose the right file coarseness for checkering, these will be used almost exclusively for cutting new checkering but probably an old checkering repair occasionally. So for cutting new checkering what would be the best "all-round" coarseness number for a 20 LPI? How about the 40 LPI?