Originally Posted by williaty

The RD won't take that much chain. Chain length will be set so nothing is destroyed if it's accidentally put into big-big and then will fall slack about half way through the cassette when in the granny ring. The goal is just to keep tension on the chain in the granny ring up to a small enough cog to get a ratio overlap with the middle ring.

26" rear wheel so I'm looking at about ~10.5 gear inches, which isn't unusually low for loaded touring in a recumbent tadpole trike. I currently have a 22T granny wit the 42T cog in back and it's nowhere near low enough.