Despite a long standing respect for SB, I have always used Shimano's recommendation of largest chain ring - smallest rear cog - derailleur pulleys line up perpendicular to the ground that comes packaged with its derailleurs. It has never failed me. In this system the chain is threaded through both derailleurs when the measurement is established unlike with Sheldon's system. I can see how the Shimano recommendation might not work so well for a very low (largest) rear cog, but as I say, I have used it up to 28 on a short cage RD without fail.