[rotrex]

1 to 1.5cc per minute per HP is a good average number.
Depending on your ignition system and how much you are then still knock limited, you can add more that that.
So for a 500HP engine, 500 to 750cc a minute is a good starting point.
Depending on the plenum design, you might lose a lot of water on the walls of your intake plenum. this makes is less efficient.
in this case you might consider injecting pre compressor and further away from the intake.
Alternatively increase the methanol content or go direct port.