I can' t say I've ever lived by this "rule". Consider this: If the chain is too long for little-little (a rarely used combination in my experience, no, make that never used) then the worst thing that happens is the return section of chain dangles loose and the RD cage may wrap around to the back. But if the chain is too short then the big-big combination will break something, most likely your RD in a spectacular way.

I guess everyone shifts differently, but except for the half-step crossover on the Masi I spend most of my time on the big ring. I have once or twice shifted down another cog, having forgotten which cog I'm on, then discovered it wouldn't go down any more. Which is to say that big-big is an easy mistake for me to make.

So just fit your chain to handle big-big, and as long as your RD cage has the take-up capacity you'll be fine.