I've found you want match the unloaded weight distribution so your bike will handle about the same when loaded. If currently 60/40 rear/front, ideally you should add weight 60/40 rear/front. Also important is to add as little weight as possible forward of the front axle and behind the rear axle. Also keep the weight as low as possible. A good test is to see if you can ride fully loaded no-handed. If you can, the bike is well balanced.