Thanks Ian - I'll have to have a go at this for my Speed Graphic!
Have you tried doing two at once instead of using a separate grinding piece? As you say, modern glass is very flat so the reason given in 1898 for not doing it shouldn't stand any more...
I thought that too but it does not work so well, principlely because it is difficult to get all parts of the GG surface grinding equally. You will need to concentrate on a few areas where your random movements are not so random usually the corners. To get the corners of two gg's going together is hard and you will end up doing an uneven job on one or both.
"There are a great many things I am in doubt about at the moment, and I should consider myself favoured if you would kindly enlighten me. Signed, Doubtful, off to Canada." (BJP 1914).