I wound up using a 10/32 screw with nuts to adjust the spacing. As far as centering it was really trial and error until it didn't throw the stir bar as often. The washer is great as it gives metal for the magnets to latch onto.
When adjusting the placement of the washer, try to look for when the washer looks most like a circle and that is the best spot. If it looks like an oval, you have adjustments to make.
I finished!! Got rid of the spacer on the fan and just epoxied the magnets to the fan hub. Even if the balance is off a bit, the magnets are way lighter than if you get off with that big washer. Runs quietly. I took pics of the control component setup. I didn't have a fan so I bought one from surpluscenter. 3 bucks. Bought a PVC box at Home Depot and scrounged wire from my old broken microwave. Paid for good rare earth magnets. Few bucks there. Radio shack for electronics. Schematic and parts shown. That should be enough to get the same parts. MSG me for more info if u need it for your build.
I used an N type MOSFET for power control. I attached a small piece of tin to the heat sink. That should be sufficient since the fan is circulating air inside the box. 10 ohm potentiometer rated for 0.5 watts. I soldered it all up. Really simple schematic that wont roast the potentiometer like it would if you try to do it in series. Besides, this smaller pot and MOSFET are probably cheaper.