its the same as mapping your impedance rise. grab a bunch of test tones. get your ammeter and put it around your speaker wire. attach a dmm into the speaker terminal. play each test tone in 5hz increments from 30 to 35 to 40 and so on till you reach about 60 or whatever. each time you play a tone write down your voltage and your amperage.
watts = voltage x amperage
resistance (ohms) = volts / amps
this way you will see exactly how much impedance rise you get at each frequency and how much ACTUAL REAL WORLD wattage your subwoofer is getting at each impedance. to see how much your amp actually puts out, find the closest impedance you can to your sub (for example if you sub is wired to 1 ohm see if you can find how much wattage your getting at 1.2 or 1.5ohms or whatever you can find that closer.) so that way you know when your sub is playing XXhz and you measured your impedance to say 1.4 ohms...your sub is getting xxxx ammount of power.