chess 960. a true measuring stick of ones skill level?

Glad you are enjoying it!

Let's do a thought experiment.

Say a person comes into chess cold without any knowledge except for the rules and a lot of enthusiasm. They decide they want to win quick by memorising openings (because it turns out they have an excellent memory for moves).

What will happen?

They will beat other beginners who don't. They will start to learn tactics that come from the favourite openings which increases their winnings further. They will start to understand more about the midgames that come out of their favourite memorised openings. However, all that they learn about tactics and strategies usually only turn up in the opening traps they have memorised for other players to fall into.

Then we put them in front of Chess960 game.

Now their memories don't work any more. Some of the ideas they learned are still useful, but the tactical and strategical traps no longer turn up any more. Now they are naked and have to play off pure skill over the board.

I'd say it is equal to a drop in rating of two hundred points? against a hypothetical 960 player. 

In summary, I'd say for beginners they should make a key decision early. Chess or Chess960? If they go the chess route, they will learn the basic ideas more quickly because Chess is simpler and so playing 960 will be harder. If they start with 960, they will have more challenge learning the basic ideas (but it really the same - just play in the center but with caveats). They will never be as good at chess as a chess player, but they will be general good, adaptable, creative, and flexible chess players.

Cheers

a great book is "my system" by Aaron Nimzovitch...the father of hypermodern chess

I think it is a good measuring stick of one's skill level at 960.