You can correct the problem without starting over. Just loosen the spokes on the short side and tighten them on the other side. Loosen them about 1/4 turn all the way around on one side, and tighten the same amount on the other, then recheck the dishing. You will probably need to retrue a little, but if you work in small increments the rim will stay relatively true. The hub needs to be centered on the axle.