I've seen conflicting methods for determining chain length, for example:
Small cog to small ring:
Small cog to big ring:
Big cog to big ring:
On SheldonBrown.com he promotes the big to big technique:
"The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer. Mesh the two ends on to the large chainwheel so that one complete link (one inch, -- one inner and one outer half-link) overlaps. In almost all cases, this will give the optimum length."
Which is correct, or better, and why?
P.S. This is for my road bike, double rings up front, 11-25 cassette, DA short cage RD.