Calculating chain length in non standard setup

[HorTs]

I want to add a 11 speed XTR cassette (11-40) to my bike 'road' using a spacer. The rear mech (long cage Ultegra 6800) would require an extension, I propose to use a Goat Link, at the front I will be using a double Ultegra 6800 50/34.

Would I get away with a stadard chain or is there a way of calculating what I would need?

 

[Dogtrousers]

There are calculators on the web (for example this ) where you measure the length of your chainstays and enter your mother's maiden name and so on and they give you the required length.

Sheldon Brown's method has always worked form me: http://sheldonbrown.com/derailer-adjustment.html#chain

Put the chain around big chainwheel and big sprocket and add in one full link (two half links) to account for the wiggle throught the derailleur.

That said, I also like playing around with chain length calculators too, cos I just do. They generally come up with the same answer as the Sheldon method though.

Edit. I've just clicked on your link to the XTR cassette. Wow. I now realise you are entering a world of weirdness so maybe my standard answer above may not apply.

 

[MichaelW2]

parktools repair website shows a variety of methods for determining chainlength