OK, I'll think. Here's what the 68 'Riders Handbook' says:
"Tighten the rear nut (illustration 6, item D) on adjusting eye bolt until, with the finger through the filler cap orifice, it can be felt that the chain is dead tight. Then slack off the rear nut and carefully tighten the forward nut until there is a total up and down movement of 3/8 in. Then securely tighten the rear nut to lock the assembly. Check the adjustment in more than one position of the chain and adjust, as above, at tightest place...."
Now how hard does one engage the finger?
Actually I like to loosen the primary until there is good clutch action and the gearbox is working nice, which for me seems to be a bit more than the 3/8" described above. Just me.