Sorry to make this chain subject awkward, but the manual states that the chain slack (50-60mm) should be measured with the bike on its sidestand, and with no weight on it, ie suspension fully extended.

For what it's worth, I set my adjustment with my bike on its centrestand, which is the same in theory as an unladen bike leaning on its sidestand. The only problem with the sidestand idea is the assumption that the suspension will be fully extended. Depending on the spring rate/preload used, this may not happen if the shock is set up to be soft.

I agonised over the chain slack question after having set it much too tight at one stage. I figure the above explanation is right after I compressed the suspension with two strong tiedowns, and then set the slack so the chain wasn't tight with the three axles/pivots as close to being in line as possible. I then released the tiedowns and measured the slack with the suspension fully extended, and sure enough, it was pretty close to 50mm.

I hope this isn't too long-winded, and makes sense. Why Yamaha didn't fit a rear chain guide I do not know . Bloody penny pinching maybe. Even a DR650 has a decent one.