Water injection and FE?

[nubbzcummins]

My water injection setup on my truck starts spraying 15% duty at 2psi manifold boost and progressively increases the spray amount from there up to 100% duty at 5-30psi(where ever I set it) manifold boost. On flat road I have to be cruising at aprox 62-65mph to get to the start spray point. So lets say if water injection will boost FE 10% will I get better FE by keeping boost to where it is spraying most of the time and traveling faster or not spraying hardly at all by keeping speed down? Example: 55mph/1340rpm/1psi/no water injected.....65mph/1590rpm/2psi/water injected +10% FE. I'm guessing that since the speed and rpm will be linear it all boils down to manifold boost pressure being to the point to where it will negate the positive FE from the water injection?