Sid here . I have always sought to have 3/4 inch mid way on the lower strand of the rear chain with the rider in the saddle . Both single and twin . The Handbook dictates this in fact . Never had a problem with this setting . Just reread PEIs writings on the subject and see no reason to change .
If a rigid frame is being used , as my drag bike was ie no rear shocks , I use a figure of 3/8 to a half inch only . If too loose the rear drum can be shattered by a whip crack effect of the chain . Saw this happen on a pukka Black Lightning , the drum turned to iron dust
during a drag race upon a hard shift into second . Poof and the drum vanished .