Pe stands for Power Entered, or the watts from the amplifier. I use either the RMS rating from the manufacturer or the RMS rating of the amplifier I intend to use.
You can get the published Re value from the manufacturer. Otherwise you can use a multimeter, get the resistance of your leads by touching them together, test the impedance of the voice coil(s), then subtract the resistance of the leads to get the DC resistance. I believe you can get Re for dual voice coils in parallel by dividing your measurement by 2. If they'll be wired in series you should multiply by 2.