The forum was put here for questions to be answered.
The amount of injection mixture you want to inject should generally be considered in terms of the mixture to fuel.
Calculate your fueling based on the pressure you are actually flowing through the injectors (since very few actually use the pressure that the injector's rating is based on). The determination of fuel rate is based on that flow times the duty cycle of the injector.
From there determine how much you want to inject relative to that fuel flow. Look at the pump pressure and the jet's flow rating at that pressure.
If you just want knock suppression and do not want to modify your AFR or timing. Usually a very small amount of water mixture will suppress knock somewhere around 5%.
If you want to remove some of the excess fuel being used for cooling currently (i.e. lean to a better power producing AFR around 12.5) 10%-15% generally has done well.
Keep in mind that if you use a static flow system - the relationship only holds at a given point - below that point you are flowing more water than you targetted relative to fuel and worse above that point you are flowing less water than targetted. I recommend static systems be sized appropriate to WOT and max duty cycle and do not engage the system too early.
A system that will vary flow based on fuelling is the best since you know you are close to your targetted relative water to fuel throughout the time that the system is engaged.
Injection before the throttle body is ideal. After the throttle body make sure you use a check valve to prevent siphoning. I have heard of no adverse effects from injecting before a roots blower. However with a centrifugal blower or a turbo you should inject post compressor.
HTH