I would suggest you focus on getting your lip and hardware installed correctly, and not try to get the lure body perfectly symetrical.
I don't worry about getting my cranks exactly symmetrical. I cut my lip slot after I cut out the profile, while the blank is still flat, all on a bandsaw.
Like Bassguy, I mark a centerline around the perimeter of the bait, and work to that, sanding on an oscillating belt sander until the shape looks good to me.
Of far greater importance to me is that the lip goes in square to the blank, and centered, and that the line tie, hook hangers, and ballast holes are centered, too.
If you think about it, only the top of the head and the back are really disturbing the way the water passes over the crank, and I can get them even enough by eye. If the belly is even to my eye, it's close enough, too.
Your eye will tell you if it's too far off. Trust it.