UPDATED
When a new value "arrives" from the mqtt in node, the change node takes the payload and store it in context in the "flow.fuel_remaining" variable
If you need that value you can simply get it as in the example below. So you would not query the mqtt broker for the value, you "ask" NR to provide you with the value stored in flow context
If you need help with NORA nodes, I know nothing about them
[{"id":"547e030e.47f42c","type":"mqtt in","z":"bf12ff9c.cd354","name":"fuel remaining","topic":"/garage/keroscene_tank_%_REMAINING","qos":"1","datatype":"auto","broker":"75eba16c.094f9","x":230,"y":290,"wires":[["7b6775c5.ce0ddc","b3dea6d4.677f98"]]},{"id":"335f4182.f383be","type":"debug","z":"bf12ff9c.cd354","name":"MQTT_fuel_remaining 2","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":810,"y":330,"wires":[]},{"id":"5e86aa8a.322204","type":"comment","z":"bf12ff9c.cd354","name":"MQTT test nodes","info":"","x":240,"y":230,"wires":[]},{"id":"7b6775c5.ce0ddc","type":"change","z":"bf12ff9c.cd354","name":"payload to \"fuel remaining\"","rules":[{"t":"set","p":"fuel_remaining","pt":"flow","to":"payload","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":500,"y":290,"wires":[["335f4182.f383be"]]},{"id":"b3dea6d4.677f98","type":"debug","z":"bf12ff9c.cd354","name":"MQTT_fuel_remaining 1","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":490,"y":330,"wires":[]},{"id":"c69dd8b5.6085a8","type":"debug","z":"bf12ff9c.cd354","name":"MQTT_fuel_remaining_3","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":1090,"y":290,"wires":[]},{"id":"37c5c3ed.7a7f4c","type":"inject","z":"bf12ff9c.cd354","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"75","payloadType":"num","x":220,"y":110,"wires":[["fd4e11b6.41cf3"]]},{"id":"fd4e11b6.41cf3","type":"mqtt out","z":"bf12ff9c.cd354","name":"","topic":"/garage/keroscene_tank_%_REMAINING","qos":"","retain":"true","broker":"75eba16c.094f9","x":520,"y":110,"wires":[]},{"id":"2facacd3.8dcfb4","type":"inject","z":"bf12ff9c.cd354","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"100","payloadType":"num","x":220,"y":150,"wires":[["fd4e11b6.41cf3"]]},{"id":"b5ff6241.e3c19","type":"inject","z":"bf12ff9c.cd354","name":"","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"30","payloadType":"num","x":220,"y":70,"wires":[["fd4e11b6.41cf3"]]},{"id":"4f7041de.eae36","type":"inject","z":"bf12ff9c.cd354","name":"Get value","props":[{"p":"payload"},{"p":"topic","vt":"str"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"true","payloadType":"bool","x":230,"y":480,"wires":[["b588ab9e.a24a48"]]},{"id":"b588ab9e.a24a48","type":"change","z":"bf12ff9c.cd354","name":"\"fuel remaining\" to payload","rules":[{"t":"set","p":"payload","pt":"msg","to":"fuel_remaining","tot":"flow"}],"action":"","property":"","from":"","to":"","reg":false,"x":500,"y":480,"wires":[["2d3a121c.6f848e"]]},{"id":"2d3a121c.6f848e","type":"debug","z":"bf12ff9c.cd354","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","statusVal":"","statusType":"auto","x":770,"y":480,"wires":[]},{"id":"75eba16c.094f9","type":"mqtt-broker","name":"","broker":"127.0.0.1","port":"1883","clientid":"","usetls":false,"compatmode":true,"keepalive":"60","cleansession":true,"birthTopic":"online","birthQos":"0","birthPayload":"BULB-1/LWT","closeTopic":"","closeQos":"0","closePayload":"","willTopic":"offline","willQos":"0","willPayload":"BULB-1/LWT"}]