Filter Rule For Ultrasonic Distance Sensor

[DaveS]

Hi All,
Please help a noob with espeasy rules ...
I have a JSN-SR04T waterproof ultrasonic sensor I intend to measure water level in a rain water harvesting tank.
These sensors also work with the newping libary.
When testing the sensor,every so often a zero value is returned otherwise readings are quite accurate. (every 7th or 8th reading is zero)
How can I set up a rule that will ignore zero values ?
Taking an average value from 10 or 20 readings will still have a large error due to the zero value readings.
I also want to convert the distance value to litres and multiply value by 2 as there are 2 identical tanks connected together.
Can all these calculations be done in one long rule or is it best practice to write 2 or 3 rules ?
tia
Dave