just add more percentage of meth. The more meth you add the freezing point goes down. In -20*F windshield washer fluid its got about 35% methanol in it, and adding more will reduce it further. There is no way that your IATs are gonna be anywhere near even -20 so id say a 50 50 mix is fine, One thing i would do is inject the nitrous AFTER the methanol into the intake stream, so keep the nitrous injection closer to the engine itself, to protect against freezing or frosting the meth injection nozzle itself which could restrict flow and prevent proper atomization. Running nitrous on the hot side of the intercooler negates some of the cooling effect of the nitrous, the closer to the engine the better. Truth is that race gas is always a better bet but i dont see why a properly tuned setup shouldnt work with boost/meth/nitrous, but race gas is safer and youll make more power with race gas. Just alot of variables here and you still are gonna need to retard timing and play with fuel, Its all in the tuning