So haynes matches up with the yam service manual, however if the slack was adjusted on the center stand, then the slack would go down, when one person sits on it normally, and then go down again with 2 people on it.
That is Ade's point, that the tension would be too high with 2 people on it for example, possibley runing the output bearing, possibly heating the chain up to cause failure etc. Its not the numbers that are being argued.
Oddly, the service manual says to check on the sidestand, the owners manual says to check on the center stand, and haynes says to check on the side stand. You can only assume that the sidestand and center stand measurements dont make that much difference (seing as the weight is still on the back wheel when on the center stand), maybe 5mm out if that?