Why don't you try what you have without magnetizing. I doubt you'll have any problems. Make sure to unplug the saw first. When you place the dado set, push the first outside cutter tight to the arbor flange and hold it with one hand, then progressively add chippers and shims as needed, keeping each one tight to the other. The shims don't have to be distributed between all the chippers unless the sum is larger than the overlap of the carbide teeth. You may be able to make small adjustments by just removing the outside cutter and changing the shim between that and the first chipper. As you place each cutter, position them so they balance evenly around the arbor and no teeth hit each other or the plates. Lay out your dado set in order ahead of time so you are ready to go and not fumbling for things. You can feel and see if things are fitting nice and flat to each other. Since the arbor threads spiral, the shims don't just drop into the groove easily, you might catch one once in a while but it shouldn't be frequently if you take a little care placing everything. Don't crank on the arbor nut, just lightly firm with any blade.