It's personal, and since you weigh feck all, I'd have it set pretty high. Depends how fast you want the shock to return, and obviously what state the shock is in. An old one (like mine) with years of abuse ended up being wound up to the end stop as the bike was a bit springy.
The real answer is to set it mid way and ride it. If you get wierd boingy feelings accelerating round corners it's probably too soft (spring returning too quickly and overshooting). If you find the back is getting compressed and not returning over lumpy track it's too soft (spring is not extending enough after succesive bumps).
Hope that helps...
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