We use the charts to find the declared conditions, add the elevation... All books state this, but....
Then what?
Back in the old days - can't give you the date things changed - every aircraft type "certificated" to operate in Australia had its own set of "Australian" performance charts. In the POH you would find the manufacturers charts, but in the flight manual would also be the Australian set, approved for use by the Australian regulator of the time (called CASA today, but in the past has been DCA, CAA, DOT).
The Australian set were specially printed, using data from "certification" flights that the aircraft type had to undertake in order to gain Australian "certification". Among other things, the Australian charts used the same distance factors across all aircraft types - the same factors that appear in CAO 20.7
Those charts allowed you to enter them directly with a density altitude. This is where those graphs of declared conditions came in.
Nowadays, new types entering Australia don't have to go through that "certification" process, and you may use the manufacturers performance charts.
Your way of using the declared conditions to arrive at a PA and temperature is the best. The other two methods are adding an unnecessary safety margin. There would already be several safety margins built into the figures you finally arrive at.