Hi Dan
I'm a bit late to this but here's my two bits worth:
I think Rohloff and Thorn are compelled to give numerical values for chain slack because it simplifies writing the manuals and takes into account users with little mechanical experience.
IF you can find and feel the tightest spot on the chain, and that point has a little slack, then this must be sufficient as the chain would need some tension in it to preload and stress the bearings.
I think getting it to the minimised slack condition has one key practical advantage: you can go more miles to the next adjustment with less worry about it falling off.
Practically this may allay the need for an in the field adjustment on some trips, therefore less to worry about.
For day to day use it just moves adjustment requirements as far apart in time/miles as possible and it implies fewer eccentric indents with the widest possible spacing.
I find if I start like this, with a new chain, excessive slack then occurs just as the chain is arriving at 0.75% stretch. I also finds chains seem to run themselves in and gain some initial elongation very quickly when new. (Caveat: do not adjust it slightly tight on the basis this will happen as it will overload the bearings. Even large motorcycle sprocket ie (gearbox output) bearings have been known to fail under tight chains.
Ahem!)
So far I have found removing chains at 0.75% stretch or so means a new replacement can go straight on on the same b/b position (but I still check the slack carefully as they go on!). The cunning plan is to keep dropping new chains on until the chainring and sprocket teeth show visible wear. At this point I plan to reset the b/b position and run through the same set of chains again and stretch them to almost fallling off (1.5% stretch??). Then scrap the chains and reverse the ring and sprocket and start again....
Happy fiddling
Ian
