In the service manual......If you go way back to page 445, which is the beginning of the rear wheel removal/replacement section, they say to support the bike on a jacking stand so the rear wheel is off the ground.
In the following pages the drawings show the bike on its center stand. The drawings also show the bike in the vertical position.
Although I agree that the are not explicit,but I would say...the 30~40 mm freeplay is with the rear wheel off the ground and the suspension extended.
The spec of 20~30 from the owners operating manual is probably with the bike on the side stand. "most" bikes, it is fairly standard, should be around 1" freeplay in the chain while sitting on the ground on the side stand. 1"= 25 mm......so 20 ~30 is about right. (dirt bikes much more).
A too tight chain is far worse than a slightly too loose chain.
Cookie
Page 83 in the owner’s manual specifies 20-30 mm free play.
Page 460 in the service manual specifies 30-40 mm free play.
I called Royal Enfield USA and got no help.
In the meantime I’m figuring that 30 mm +/- is what I should use.