What I have found is that with the chain properly adjusted (on my bike anyway) it will just barely touch the bottom of the swing arm pad. This is right at 25mm. I prefer to run my chain a tad on the loose side as I am AHEM... a bit on the large side. A little loose is FAR better than a little tight.
Also, depending on what kind of chain lube you use, always check and adjust the chain BEFORE you squirt on your favorite slick stuff. Using PJ1 myself, after I adjust the chain I lube it and once the lube has set up, I have only about 7mm of slack on the chain. However after riding for about a mile, say to the local Stop-N-Rob, the chain is back to normal.
