But, astcyr, YOU didn't actually say, "What is the correct slack". But you gave a "general" concern that we all have. Don't mean to beat you my friend, but the whole idea of the post is WHAT IS THE CORRECT SLACK? My whole idea of sitting on the bike would give you a more precise slack measurement, during operation, with your weight on the bike. Even the guy on youtube(forget his name) that takes a screwdriver, and lifts the bottom chain till the top chain starts to move, and he says, "Thats your slack". To pull the chain down, with the chain weight already pulling the chain low and then pulling the chain back up and saying, there your slack, might be 100% correct. And the guy in the youtube video is a motorcycle track instructor. AND, he does it without sitting on the bike. Now this is MY BIKE only; When I take a measurement sitting on the bike(upright), and take a measurement just on the stand(without sitting on the bike), I have a 1/4" more slack measuring on the stand. I have to think that sitting on the bike, with my weight pulling the tire backwards, is a more realistic measurement.
Isn't what your trying to accomplish, is to see what the correct measurement(slack) is during operation, with your weight on the bike? Maybe both of our ways accomplish the same thing. From what I've read(manual), the 10-15mm; Most people have said that is too tight, and 20-25mm is the correct slack. I can't help but think, that out of ALL the racers/bikers that someone would post the CORRECT slack for such a simple setting. My brother says 3/4", and with his "decades" of racing/cruising (etc), I have to go with him.