How to set a max length from a drawn Line

Hello Stencyl Community!
I just downloaded Stencyl yesterday and i really like it so far.

I have a little problem i don't know how to do and i didn't find any solution yet.



The image on the left shows what i currently have. But now i would like the line to stop after some length  (as shown on the right).

I uploaded my file to this post. The number in the middle is the distance between the white dot and the mouse.

Would be awesome if anyone can help me with that.
Kind regards and hopefully you understand what i would like to archive, even though my english is not perfect.

You can do it using maths, but maths isn't my strongest area, so I can't tell you. Maybe google it.

Alternatively, use the drawing extension: http://community.stencyl.com/index.php/topic,35352.0.html

Something like this is what I'd try, without using extensions.