If you want the longest possible chain, use the little/little method and shorten just enough to cause the lower pulley to move forward a bit when the ends of the chain are brought together. With this method, the chain is threaded through the RD and positioned on the smallest cog and smallest chainring.

What the cage looks like in the big/big is irrelevant. If by "directly below", the author at slow twitch means directly below along a vertical line, that's wrong. That's often the description for the position when the you're in the big ring and smallest cog, which is not even close to the maximum extension.