Water and AFR

[sdminus]

I am about to tune my rx7. I am gonna do a base tune at 11.1:1 afr. (rotaries like it rich) AFter the base fuel map is set up i want to take full advantage of my WI system. I am still having toruble deciding how much water to add in realtion to fuel. I have seen people talk about subtracting fuel and replacing with water like for like. Is this true?????.
Is this the reason why whe n water is added performance drops????
I am of the understanding that we use water to replace the fuel used to cool the combustion. But water is more efficent at heat absorbsion. I am told 3 times better than petrol. With this inmind can i say pull fuel and replace with 1/3rd water???
can any body enlighten me and save me from an expensive mistake

Scott