For this intercooling purpose if you have a valve in the WI system that can be driven as you describe based on temperature then you will be sure just to inject enough water to maintain the desired intake temperature. That would give you good reference for intake temperatures when programming your fuel without worrying about too much water. The temperature you are seeking is up to you really and may be a matter or trial and error. Also the exact IDCs and flow rates will take some trial and error. Without mapped water injection you may be injecting going well above and well below your target temperature rapidly and the on/off cycling of the WI may in that case cause inconsistent engine performance if it is more water than needed. I hope I have explained this in a way that you understand what I mean.
There is not solid rule on what boost level to inject on. The 6 psi reference may be for turbos and that level would be for a couple reasons. Turbo engines frequently have lower compression ratios and 6 psi would be a good measure to prevent running the WI during relatively low loads when it may not be necessary. Some GTP users for instance do not run much more than 6 psi of boost and are using fixed pressure switches that engage WI at 3 psi.
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