So, first of all, I'd like to point out I've done way too much research with this, I make my daily coffee with holy water for funsies, holy water blessed by TWO popes. I've looked into catholic websites, interviews, read through every relevant bible passage. Trust me when I say, I do not know for sure. Every source that even approaches this little bit of nuance seems to assume a pour is instant. In a way, that makes sense, because we cannot dilute it. So, the reality I've concluded is that we have to do the same. Therefore, flow rate is irrelevant, if I take a whole hour to add a liter of water to 999 ml holy, I ruin the batch.

Instead, the device I've imagined (but not tried to build, as of yet) to continually produce holy water would use essentially a mini 1-liter toilet tank setup with a reeeeaaaallllly slow drip emitter as an input, so that the "flow" isn't continuous, it's discrete droplets being added to the holy tank, and the bobber that toilets have is set to keep like 100 ml as a minimum to this tank. Then we prime it with holy water and every drip is less than the size of the tank, so holy water is constantly growing, and then whatever output I have can only get holy water and can never deplete the tank.

Also you can't pour holy water down the drain, that's a sin, it has to go into the ground directly. Churches have a dedicated holy water drain that isn't connected to actual plumbing and goes straight to the ground. Following the rules for that is actually quite annoying.