HI,
It all depends on which stand you use , rear or centre. For a d type stand 1.5 inches is correct, but it does say after adjusting to put the machine on to its wheels, and check that it has no less than 3/4 slack at its tightest point. When using the rear stand 3/4 is correct.
for detailes see Paul richardsons book vincent page183.
Personaly if using modern o ring chain i would tend to give them a bit more.
stumpy lord.
p