Density is just pressure corrected for temp, so the first method would be the correct one if you are using the figures off the declared density altitude charts. If the temp has no deviation from ISA, the PA=DA.
These charts are (same as your take-off question) worst case scenario and are used where a TAF or observed conditions are not available.
If you are at the aerodrome, setting 1013 on your altimeter will give you the pressure altitude and you can get the temp off the aircraft's OAT gauge.