Joking aside...

You'll find you won't be able to get much more than 40-45mm before the chain touches the bottom of the swingarm as you have found.

What I tend to do is adjust it till it hits the swing arm and then give it a touch more slack, tighten everything up, get the bike off the stand, sit on it and check again. Basically it's a best guess. As long as the chain doesn't get too tight when everything is in line and it's not so slack that when the suspension is extended it is at risk of rolling off the sprockets, you're good to go.

At the end of the day though, it's down to guesswork and practice - you'll get a feel for the correct tension with experience. Also bear in mind that changes in rider weight, preload, linkage bone length, shock etc. will cause a deviation from "book" measurements. My Z has got +15mm links and a Yacugar shock and the slack required for smooth running is somewhat different from stock.

To get a better understanding of what's going on and get a feel for the slack needed, you can (if you can be bothered) ratchet the bike down to get the rear axle in line with the output shaft (the chain's tightest point) and then check there is still some slack to give you a reference point.

You'll clearly notice if it's too tight as the whole drive train will be noisy - there will be a pronounced whirring noise. Also the suspension may seem stiffer than normal as the chain will be adding resistance to smooth movement. Too loose and you'll notice sloppy take up from a closed throttle and jerky transmission, plus you may hear the chain slapping against the swingarm or chain guard.

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