Just take chalk and made a line across the tire. Drive straight for 200 feet and if it wears in the middle only take out 2psi and repeat until it's about almost to the edge. If it all wears away add air. Always better to start over filled (ex. If usually 33psi start at 40psi) so you don't have to worry about adding air just letting air out.
YouTube it for an example.
YouTube it for an example.