Originally Posted by apodysophilic
I replaced a Shimano Ultegra 6600 with the same kind because it was worn out. So I guess what's the extent to which you can cross-chain..upto last-but-one OK?
There really isn't a correct answer to this. Everyone will say never go big-big or small-small. It has to do with the number of cogs and the length of the chainstays. If you have ever noticed, a chain does not angle to the side very easily. If you have very long chainstays, then the angle of the chain will be less than if you had short chainstays.
What you want to do is try to have as straight of a chain as possible. If moving from the small chainring to the large and moving to a larger rear cog will give you a better chain line, then you should use that and vice versa. On my 24 speed, I will never go to to the second biggest or second smallest cogs with my smallest and largest chainrings. The middle chainring will never go to the biggest or the smallest cog. Basically, I will only use 6 of the 8 cogs with each chainring.
Depending on your set up, you most likely have some overlap in gearing. Use Sheldon's (pbuh) Gear Calculator
to discover your gear inches. You can print out a small cheat sheet to tape to your stem. Once you know what gear inches, gain ratio, etc. you want, determine which chainring and cog will give you the least angle.
If you want, you could even black out the combinations that you don't want to use. For example if you have two options that will give you a gear inch of 40.8, black out the one that has the worse chain angle. I haven't done it yet, but I want to set my cheat sheet with blacked out combinations that I never want to use, highlight combinations in orange ok combinations and yellow for best combinations.