It is also good to understand that each camera gives slightly different sized borders. I use a Leitz Focomat 2c to print 6X6. The negative holder is 6X9 with two glasses. The other day I could simply not get any borders with a negative from 1990. This negative was too large and I believe I used a rather cheap Ikonta folder, perhaps a Nettar. A friend told me the older Zeiss cameras give large sized negatives. Like with the Beseler holders there is not much room for any filing in the 2c holder, but it was sufficient to file only one long side, about 1.5 to 2 millimetres.
Over the years I have filed out many holders, in particular for the Focomat 1c (135mm). It is correct you will never be precise. I accept that and file out a little more than needed, and then use the easel to create the border I like. In other words the blades of the easel determine the width and the look of the outside of the borders. The other way is to have an enlarger that projects the negative including the border and, as far as I know, the outside lines of the border will be fuzzy. Some like that, some not.