If the lower left wing is 0, but the lower right is +.5, then the wings were not joined straight or the right wing is warped. Sanding will not cure either condition. If the wings were joined twisted, then you're looking at cutting the wing in half and rejoining it straight. If the wing is warped, you may be able to twist it back with some ammonia sprayed onto the wing and a lot of weight to hold it straight while it dries. Do a search here on the site for warps and you will find a lot of info on how to deal with them.

John