@jkandasa Thanks. Got it working.

@jkandasa Thanks I have already done those steps. The issue I have is trying to set a value for the virtual sensor via javascript.
What I have done thus far is

1. Create 2 virtual sensors
2. Create a "operation" script - using javascript
3. Create a rule that triggers the script when value changes

I've added my javascript here, but the last 2 lines of code are not setting the virtual sensors values. Can you assist with how to do this please?

// Get Pump sensor via UID-Tag
var tankDistanceSensor = mcApi.uidTag().getByUid("water-tank-distance").getResource();
var tankLitresSensor = mcApi.uidTag().getByUid("water-tank-litres").getResource();
var tankPercentageSensor = mcApi.uidTag().getByUid("water-tank-percentage").getResource();

//get config in variable
tankConfig = mcApi.variable().get("water-tank-config");
fullDistance       = tankConfig.value; // Value 
emptyDistance = tankConfig.value2; // Value 2
totalLitres = tankConfig.value3; // Value 3

//Get sensor values with UID Tags
var pingDistance =  tankDistanceSensor.value; //Water tank level percent

if (pingDistance > 0 && pingDistance < (fullDistance + 50)) {
  percentage =  ((pingDistance - emptyDistance) / (fullDistance - emptyDistance) * 100);
  litres = percentage / 100 * totalLitres;

  //THESE 2 LINES DO NOT SEEM TO SET THE VALUES  
  tankLitresSensor.setValue(litres);
  tankPercentageSensor.setValue(percentage);

}