calculating distance between 2 points in video

I’m working on a project that detect a moving object on a video and let the user to specify point in the video, then the system will calculating the distance between the detected object and the specified point, if the object become near to the point an alarm will triggered..

My question is how can I chose the threshold of the distance? how can I know the distance value that means the object is close to the point?

I chose an arbitrary values but it dose not always give me good results.

[awezmm]

I think the definition of a close distance would be different for everyone, wouldn't it be up to choose when to trigger the alarm? There doesn't seem to be a right answer.

[jory j]

Thank you