I am fairly new to Node Red, have some experience with RPI and Linux though, so I used Peter Scargills script to install Node Red and lots of other utils and all is working fine. I am now in the process of adding sensors to my MQTT server to help run things and make my life easier...
I have a hardware node on my system which outputs litres per minute from my utility (flow sensor), it also controls an open close valve (nano with ESP WiFi, flow sensor, relay and valve) and every 10 seconds updates MQTT server with flow rate). What I want to do is to use the HTML number input node (in Node Red), pick the amount of liters I want to fill the tank with, this then sends ON to my MQTT server (utility water flow valve) and then Node Red measures the flow, adding it up as the water flows, then sending an OFF to my MQTT server (utility water flow valve) when the amount I chose is reached. Bear in mind that the tank may be at any level when I want to use this feature (I also have an MQTT node that measures the water tank fill level (PTZ water pressure sensor on another nano with ESP WiFi) and another to measure the output pump of the water tank (with pressure and temp))). I am handy with a soldering iron and Arduino's, but not Node Red, yet... :-).
I have added a pic of what I have so far...