I believe Fox recommends 15%-20% sag for most of their rear air shocks. Here's the online manual for one of their new shocks, but the steps are the same for setting up sag: http://www.ridefox.com/help.php?m=bike&id=68#settingshockairpressure. The exact PSI you end up with depends on how plush/firm you want the rear shock to be, but a good starting point is 1 psi for 1 lbs in weight of the rider (including riding gear). Then just add/remove air to your liking.
a good starting point is 1 psi for 1 lbs in weight of the rider (including riding gear).
Well that's not necessarily true and that is why I was asking which model we were talking about. For Float shocks (without a cartridge or spring), that would be the case. However, that rule could be quite damaging for a spring equipped shock. For example, if a 220 pound rider puts 220 psi in a DHX RC4, he would do permanent damage and void the warranty. RC4s are "air assisted" and require less pressure than a true air shock. The necessary range for an RC4 is 125 - 200psi.
At 75kg (165lbs) you will probably want 130ish in an RC4. 165psi would feel pretty harsh. However if it's a float, then 165psi would be an appropriate starting place.
As mentioned above, the best way to determine appropriate psi is measure sag. 15-25% is appropriate.