Creating a reference line among LiDAR points and retrieving distances between them?

I have a LAS dataset which contains points with x and y coordinates. For further analysis I need a new dimension which can be adjusted as the dataset requires. To make it understandable I made this (beautiful paint drawing):

So in this case I need a V values which shows the distance along the V (North-East; South-West)

How is it possible to create these values?

I'd like to do it in Python, but any other working method is welcomed.

(I have ArcGIS 10.2.2 [built in 2.7 python], python 3.5)