Ultrasonic sensor - Offset Distance

Hi, I’ve got an ultrasonic sensor to monitor my salt bin. The setup all works well but my sensor is 10cm above the highest level the salt will be so I need to program that in. I thought this would work but doesn’t?

What am I doing wrong, please?

- platform: ultrasonic
    trigger_pin: D7
    echo_pin: D6
    name: "Saltlevel in cm"
    update_interval: 60min
    filters:
    # I multiplied by 100 in order to get CM since the sensor works in meters
    - lambda: return (0.45-x)*(100+10);
    unit_of_measurement: "cm"

Any ideas anyone?