With split grade printing would it then be correct to use the single filtration method as logically grade 0 and grade 5 are only using one colour.

For spilt grade I just turn the knobs all the way. No need to worry about what the exact grade is for the two exposures. At least I doubt there is. FWIW when I tested my old durst I got much lower then grade 0 with full yellow. At least that's what I remember.