Hi CME,
I must admit that it does seem a 'hit and miss' affair and it is surprising just what a difference one link makes. I found that as long as the two sprockets were in dead alighnment, the 'slack' side of 'tight' was fine.
As for pinning the sprockets down, I drilled a hole tapping size for 12BA and then screwed a short - 12mm long, steel bolt through which seems to have done the trick. The attached picture is the closest that I have for a delrin drive, but I hope that it shows the 'tension' ok.
[attachment=0:1o1hc7h5]IMG_0596.JPG[/attachment:1o1hc7h5]
regards
Mike