7 / 8 inch by 20 tpi certainly ok . Really it doesn't matter that much but there are just a few considerations :
Nut and associated washer need to run up to the cutter nice and square so as to get an even clamping load – this really means arbour and nut need to be be screwcut if possible – so choose a nice simple thread pitch .
Not usually a problem if you are running cutters the normal way round but consider provision for a locknut if ever needed .
Michael Williams .
PS : In happier times all sorts of machine shop sundries could just be bought – including standard cutter arbour nuts and washers . Try WDS or similar – you might get lucky .