Length of digital cable?

Picking up where I left off a year ago, somewhere on the net I ran into a recommended length of 18 ft of digital cable between transport and D/A converter. Various lengths were discussed and one source was very confident of the 18 ft length to successfully resolve jitter. And the Beldon 1694A was recommended. For $30+ or whatever I gave it a shot and was surprised by the quality of sound, and I swear on a stack of owners' manuals that this length with this brand of cable resolves jitter. The sound is rock solid gorgeous.

The Beldon lies behind the system mostly in a coil, and I think coils are probably not ideal. I'm wondering if anyone has found a shorter length that works. Al suggested 8", but I was unable to make it work because the components aren't close enough. I know that 1 ft" doesn't work and that 1 meter doesn't work.

Part 2: Does length of ethernet from computer to avr also affect jitter? My sister who is a computer person and not an audio person set up my avr with computer using cat 5. The sound quality is bad. I'd like to get good sound quality with computer as source down the road but need advice on how to get it.