firstly, you need to specify on which netlist you are going to perform the approximation? i mean whether you are performing the PTPX on Gate level netlist or pnr netlist? As the both have its own way of doing it.
Another point to note here is PTPX is just an approximation of the power that you chip/core will take/consume. Thus, Unless and until you know which block(if you are performing chip level) or which part is your core is being utilized maximum at what time. Thus, it is always advisible to perform Average power analysis or so called time based power analysis. In time/Avg power analysis what we do is we take a window where "WE FEEL" the toggling is maximum through out the design for a specific time- 3 milli sec is morethan enough depending upon the complexity of your design. Then we perform the analysis. Why it is called average/time based because, Toggle rate is defined as the number of toggles per clock period. so it will calculate in this manner. Your window of interest's toggling rate will be divided by the clock period.
Where as in peak power we directly take the peak consuming window and perform the analysis.
cheers.