Timing at peak torque will depend on the fuel you're using, boost you're running & where it comes in, and whether or you have built engine or not and would like to push things. If you have access to a dyno, you can start with a lower timing at peak boost and increase gradually to see what gains it gives. If you turbo comes in hard, say under 4000 RPM, be a bit careful adding a bunch of timing at peak boost.