Vitz tank capicity is around 40 Ltrs, so first fill 500 Rs (aprox 6 Liters of High Octane) and fill remaining tank with normal (It would be around 35 Liter Normal and 5 Liter High Octane).
It should remove your knocking issue, also reset your ECU as well by removing battery terminal for around 10~15 mins. So computer can adjust itself according to new fuel.