Originally Posted by
cxwrench
The safest way is small/small w/ just enough tension the chain doesn't rub on the upper pulley. If all parts of the drivetrain are in spec this will always work. Some of the new 1X drivetrains use different methods but this works on a double or triple every time. There are no formulas to determine chain length. As mack says in the post previous to mine "the only way to do it right is to do it on the bike". If you've watched 'several' videos you really should be able to figure this out, it's not rocket surgery.
I do the small/small method as this gives a little extra chain length than the big/big method and I tend to prefer a little more chain slack/less chain tension in the system. I don't think there's a mechanical benefit to it and there are likely some drawbacks, but I think the drivetrain generally feels a little smoother to me this way.