Measuring velocity with PMW3901 and RPI

So I have purchased the PWM3901 sensor from pimoroni and have been using it to try and measure the velocity. I am however running into trouble with the calculations of the measurement. I inquired about the measurements with a similar developer and they told me that by simply multiplying the delta x or delta y * height would result in the velocity. However in my testing setup this seems entirely false.

My question basically boils down to: How do you measure the speed of the surface when using a PWM3901 and distance sensor?

I have added a picture of where I added some code to display the current measurement I am doing. I am moving the sensor in the horizontal direction when pointing to a wall which is 1 meter away from the sensor.

Any input is appreciated!

I have used the pmw3901-python/ at master · pimoroni/pmw3901-python · GitHub example when making the measurements by the way. The image added above is only a sample just to show how ridiculous the current measurements were.