Not a GW but I'll give you my 2 cents. As a general rule of thumb I size up 2 shot sizes. Steel also patterns a lot different than lead, you need to pattern your gun to see what the pattern density and POI look like. I've had no issues killing big ducks with steel 4s out to about 40 yards through a LM. I've also found there's a balance between payload and speed. You can increase the theoretical lethal range of a pellet by increasing the initial velocity but the trade-off is payload and at some point you lose so much pattern density it's not worth it. I've read of guys pushing 7/8 oz of shot to nearly 2000 fps but I haven't seen any fantastic patterns from those loads.
