I had a similar situation w/ my CoMotion Triplet. The midshipman's position also had a child stoker crank attached to an auxillary BB shell clamped to the seat tube. The timing chain from the front went to the eccentric in the middle, which was always compromised between the chain running up and the one running back. One was always too loose! The "magic" link # worked, but with any stretch at all was no longer ok. I only went by symptoms after that, and learned how to forget about worrying about chain tension look or test.