Ok So I think I got it. I was going about this wrong. I can hook a pot up to my arduino and tell the sketch to use the read as the max value. This will take some changes in my sketch but in the end I think it will work as desired.
Here's my source. Except mine would be a 20W LED on a MOSFET.
http://owenmundy.com/blog/2010/05/fading-an-led-with-pwm-and-a-potentiometer/