"find out what birds are doing on your water" is the best advice you could get for decoy placement.
What we find is it really depends on species where they will tend to sit. We see redheads, wigeons, bluebills, and buffleheads all sitting together, loosely grouped amongst coots more out into the main channel of the river, often drifting down stream with the current staying in the line of floating debris from moss breaking loose. We have some very fine almost sand like snails here that they feed on a lot from what we have found in their crops. I always check out the birds crops regularly to have an idea what the birds are keying in on. The mallards we see in our area are always and I mean always tucked up against the bank, within just a few feet of the bank, so that's where we put our mallard decoys. I'll put my divers and wigeons out at 15-20 yards tops, but I want them visible for a long distance, when birds come in to land, they are always within 20 yards and in a back eddy when we shoot. Our set up of decoys is always up stream, the birds will almost always land just below or just on the edge of the decoys, then swim in to their own species if we allow it. Goldeneye love canada geese and will often land with them before swimming out on their own. Canada goose singles and pairs will also key in on goldeneye and land with them.
On lakes, which I rarely hunt I'll have the decoys as much as 30 yards from the shoreline, very few of them tucked against the bank and again all loosely grouped, with some over lap of a few species here and there. I'll always have a well defined shooting hole and a group of stoppers to put the birds directly in front of us.