sub/amp matchup

Well I had a Rockford Fosgate P3SD4-12 sub which is a 4ohm DVC shallow sub with 400 watt RMS handling and 800 max.  The amp I got for it was a Kenwood KAC-8105D with 350 watt RMS output at 4ohms and 500 watts RMS output at 2ohms.  Well the only way I can wire my sub is for 2ohms or 8ohms.  I don't know much about this stuff but when I bought everything I was told that you wanted your amp to be able to put out a little more power than your sub.  Now I'm being told that your amps rms output should be anywhere from 75-100% of your subs rms handling power, and no more than 100%.  So I'm wondering if there is a way to wire my sub and amp in the 2ohm way and then add resistors or something to possibly get 4ohm resistance (again, I know nothing of how this wiring stuff works), or if I just need to find another amp for my sub or sub for my amp?  Or do you think I'm fine with what I have?  It sounded pretty good ot me when I had it in my F250 but I had the amp turned down to about 70-75%.

I guess the main thing I'm wondering is, what should my amp rms output be in comparison to the subs rms handling, and then how much are you supposed to have the amp turned up once you get to that point?

the truth is that there is no set rule about how much power you can give a sub because it depends on the quality of your equipment and how you use it. if you have your sub wired down to 2 ohms and your amp is sending it a true 500W then you should still be just fine as long as your settings are not way too high on the amp and you know when to back off the volume knob when you start to hear distortion from your sub.