The problem with the Shimano method is the pulleys may always be off vertical - angled a little backward with a slightly long chain and a little forward with an inch shorter chain.
Campy's method recommends the longest possible chain that will never need to be lengthened if a cassette with a larger cog is added later. Thread the chain through the RD and put the chain in the little/little combination. Make the chain as long as possible - just long enough to swing the lower jockey pulley forward when the ends are brought together. Also be sure that the chain dues not rub on the pulley cage. The only time this method does not work is with a setup that exceeds the RDs wrap capacity.
The big/big plus 1 inch method gives the shortest possible chain. Users also need to know that if the ends of the chain that are brought together when wrapping the big/big are the same and can't be joined, then you add 1-1/2 inches or 3 links to get the minimum length. This is the best method to use if the setup exceeds the RD's wrap capacity, but would also result in the chain hanging loose when using the little ring and some of the smallest cogs.