"ditto" on Marks note - the place to start, is at the start, ie: 2 1/2 drams and 1 ounce, then test pattern at the maximum range you might need, then increase both shot and powder in increments up, to max. book specs until you find a load that will work for you, in that gun. With a double, it is common to have one barrel outshoot the other as to range (pattern quality).
Working up a load for a shotgun can be just as tedius as working up a load for a rifle - sometimes even more difficult. Much depends on whether the gun is choked or not - condition of the bores, etc and quality of materials,wads used.
With open chokes, some guys only use the overshot wads between powder and shot, as the thin lightweight disks may disrupt the patterns less than heavier wads.
With a choke, the best patterns are usually found with normal wad columns, overpowder, then fibre, then shot, then overshot disk wad. Some guys soak or wet the fibre wad with Neetsfoot Oil - while some shoot dry.
Track sells lubed wads and these work fairly well in my 20 bore.