Nominal = named. In other words, if you use 75 Ar 25 CO2, its safe to assume the mixture actually has those percentages for testing purposes. According to 4.5, a change in that named mixture percentage, as in your example, is a change in an essential variable and would require qualification of a new PQR which you could combine with the present 75/25 PQR and roll into the present WPS, or use as support for a totally new and separate WPS.