How can are measure the exact range of focus of a given fixed focus webcam?

By moving objects around the camera I can see that they get very blurry at 10cm and less sharp at 3-4m away from the camera but how can I measure the exact range of focus? If I check the sharpness of document at different distances – when it’s too close it’s blurry but when it’s far it’s not sharp probably due to the limited resolution.

(Actually this webcam is manual focus so basically I want to calibrate it).

I only know the horizontal view angle (60 degrees) and can guess the sensor size (1/2.3-1/4.5″).

I am also curious to know whether it’s possible to measure focal length or