Its not uncommon to have warp in an aluminum build plate. Worry about getting the three outer ones the same and don't worry so much about the center point. If you want to get a completely level bed, then invest in a borosilicate glass build plate. Flashforge USA sells one here.
http://www.flashforge-usa.com/shop/b...ate-glass.html
If you decide to get a glass bed. You will have to add a z axis shim to compensate for the added height the glass adds to your bed. If you have a .25" glass plate, then you need to add a .25" shim. Here is a printable shim,
http://www.thingiverse.com/thing:440022
I'm new at this, so hopefully others can chime in as well.