Good to see you exploring into water injection system. I have been playing with water/methanol injection and vaporization for a while now. Water injection goes before carburetor while water vapor goes into intake manifold (vacuum). With water injection, we can feel the torque improve when press the throttle compared to without. Water mist nozzle make the water breaks into mist before entering into carb. I haven't install solenoid valve to control the water injection yet (no time). So I have problem controlling water during idle but should be able to automate it with help of solenoid valve. Your method of using intake manifold and uses 0.5L per 100KM should be the way to go.
While for water/methanol vapor method, I use 1/2 methanol and 1/2 water and pour into house hold water filter container. Outside air sucked into bubbler using vacuum manifold to vaporized. It does increase the engine rpm during idle. Looks like it behaves like fuel vapor. The advantage of using water/methanol vaporization and fuel vaporization is the water/methanol vaporization lasts longer. Fuel vaporization needs fresh fuel every time to work efficiently (a bit hard to provide fresh fuel every time into vaporiser container).
(Knowledge without action is useless, action without knowledge is foolish)