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[Bertie the Bullet]

An elderly friend of mine who has been a bike mechanic all of his life taught me to adjust to around 2 inches (approx 50mm) of slack when on the centre stand, I haven't killed any sprockets or gear boxes yet so that's how I will always do it until proven wrong.....just enjoy the ride..

[m1ks]

Most important thing is that the chain isn't too tight when at it's longest reach, (this is when the gearbox sprocket, swingarm pivot and rear sprocket centres are aligned), to check this, unbolt your shockers top or bottom, jack the swinging arm up, (bike on centre stand), till straight and adjust to give a MINIMUM of 10-15mm slack between centre pin to centre pin, (or top to top, or bottom to bottom, you get the idea).
Refit and recheck and most likely you'll discover this equates to about 30mm of play with the rear wheel unloaded, (or whatever your manual states and if it states rear wheel unloaded or loaded, (on stand or freestanding on wheels)
Main thing is, if it's too tight, you can rapidly wear out your gearbox output shaft bearing or the shaft itself, =annoying strip to replace, Also, the chain, sprockets, rear sprocket bearing/carrier etc, but those are quick and easy to replace.
Of course too loose can give excessive drive lash, snatchiness and in worst case scenario jumping off the sprocket, but that's better than t'other way.

[p]

Les H and M1ks are absolutely right, you must ensure some (minimal)slack at the very tightest point, IE with swinging arm in exact direct line with sprocket centre. Bouncing/heavy friends unlikely to get this just right as the chain does indeed slacken again either side of the critical point.
To do this without disturbing suspension I used a strong strap (old seatbelt or rope is fine) tied to each side of rear wheel axle, up over a car jack on a bit of wood on the seat - raising jack squeezes suspension until correct alignment is reached. This way you can rotate wheel to find tightest point at the same time. Of course you only need do this once, get chain set right then release suspension and check slack with rear wheel off ground..... and then use this amount of slack for future.

[Howard 612]

Without negating anything that others have said...

The procedure I gave early on in the thread is a universal one that works on 99% of road bikes with regular swinging arm suspension. It was shown to us by a Police Class 1 Pursuit Rider on a safe riding course I did about 33 years ago and has worked on every bike I have had in that time. That's why I always give that procedure out as a very good "rule of thumb".