As per my experience the entire tail length is equal to entire body length, it is variable to age of the mice.

The short answer is 'no'. Mouse tail to body length varies with age, and varies between strains. If you use a single strain at a specific age the relationship might be close, but why? Mount a ruler upwards on your desk. and hold the mouse by the tip of the tail until his nose just touches the desktop. Read total animal length at tail tip.