It is impossible to give you the answer you need without knowing the pressure and flow rate of you water/methanol pump and your fuel consumption rate. I would just start with the smallest nozzel you have and work up slowly until you experiance any sort of abnormal performance, then go back a nozzel size.
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Vectra 1.9CDTI 235whp/380lbft (270bhp/405lbft) NOS+Methanol
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