Always on the centre stand or race stand, never on the lean stand.
Always on a warm, cleaned and lubed chain.
I make a point of making sure it's never tighter than mid range, target mid range, and if a wee bit slacker than mid range, it's OK.
Yes, as it's Total Slack not just Droop Slack that make up the required measurement.
A clarification to 3 above.
While the chain does not touch the swingarm with top slack, it does touch the rubber guide/protector.
I just did a garage check to get a sense of how much the chain slack changed by taking the bike off the centre stand, with the bike fully on its wheels, and not even on the lean (side) stand.
There was no apparent difference.
Also keep in mind that the 919s lean stand results in fairly significant lean angle, with the lean stand foot transferring quite a bit of weight to the ground.
Doing chain adjustment on a lean stand is at best a flawed method.
Just because a 919 only comes factory fitted with a lean stand, and has no swing arm spools, does NOT at all mean that lean stand adjusting is required, good or best.
It merely means that within the confines of just having a lean stand, use the lean stand to adjust the chain.
Finally, the 919 Owners and Service Manuals, as well as the Common Service Manual are all remiss by not even mentioning tight spot(s) checking, nor the difference in slack that occurs between warm and cold for dirty chains lubed with very high viscous lubricants.