[rotrex]

Peter,
it is a fair share of power from 1800cc inline 4 in a Datsun 1600 car.

I'd up the water content to 50% by mass at these boost levels.
The higher the boost the better water works.
The direct port injection gets around the distribution issues with the normal central injection.
Methanol will work in big droplets or stream, water not. Water needs the proper distribution to work.
50:50 by weight will likely allow higher boost with the same total mix flow compared to 80:20.

If you get misfires from more water content, you need a better ignition set-up such as a CDI system that Peter aka Riceracing has developed as he was pushing the limits of his rotary engine.
It is established for a long time now, that given you properly inject enough 50:50 mix and your mechanical system and ignition copes, you can run truely silly boost levels.