my program allow user to draw lines and the user should know the length of the line that he draw in centimeter (.cm)

I did use

SetMapMode(MM_LOMETRIC) function to convert the device units into logical units

and it works correctly but I don't know how the line length will appear to the user to let him draw in correct length

I think to make the length appear on the mouse when the user draw but I don't know how I calculate and display it

can you please help me with ideas to do that

thanks a lot