To get timing set to the absolute optimum for your car, it's an individual thing. A great deal depends on your heads, chambers, and overall combustion efficiency of YOUR engine. Generally speaking though, for D-port heads, I'd go through a "trial and error" process starting at about 34 degrees TOTAL, then creeping up from there on degree at a time until you find the point where the motor quits liking it. "Not liking it" is defined as either 1) performance quits improving and starts to fall off or 2) you get into detonation.
Finding that point implies you've got a way to check total advance and also a reliable, obhective way to measure performance --- something like an engine dyno, chassis dyno, or accurate set of e.t. clocks. The 'seal of your pants' will lie to you.
You can check total advance with a light like this one:
Actron/Digital advance timing light with LED screen and mode indicator lights (CP7529) | Timing Light | AutoZone.com
or something similar.
To check total advance (for performance purposes) disconnect and PLUG any vacuum advance connections, increase RPM while watching the light unitl you're SURE that all the centrifugal advance in the distributor is "in" (the timing marks have quit "moving") -- usually somewhere in the vicinity of 3000-3500 rpm.
If you don't have a way to accurately and objectively measure performance, then D-port Pontiac's "usually" like to be somewhere close to 36 degrees or so total at WOT with the advance "all in". One exception is 67's with closed chamber 670 heads - they often like "more" because their combustion efficiency isn't as good so you have to start the fire earlier to get the best out of them. I've heard that aluminum heads can like more advance too, because they run cooler and that too has an effect. I don't have any first hand experience (yet) with them or "usual" numbers though.
Once you've got the motor "happy" by setting total advance, then you can check it at idle and see what the initial value is (vacuum disconnected) for reference - so the next time you need to set timing you can do it at idle "the easy way".