Sounds like a complicated method ... especially considering the variations in bird size and muzzle size and length. Plus it sounds like you will have to be `swinging' on the bird(s) to see.

If you are hunting where the birds will come to you - you might pace or pre-range some distances where you will actually hunt. Mark (at least mentally) 30, 40, 50 yards, etc. And when the birds arrive you know the ranges.

Waterfowl can tend to look closer than they are, especially in flight. And it seems also to depend on the weather and light.

If you are hunting in a place where you go to them ... jump some birds, and then pace or range how far away they were.

You could also do the same thing at a city park (except for the shooting part).

But after shooting for a while - you should develop an intuitive sense of the distances (and circumstances) within which you can hit them hard. This is what I consider `in range'. And I let the other birds (beyond that) go.

I agree with Serious Hunter, it's mainly intuitive and comes with expereince. If you see enough ducks, you will "know" when a bird is in range. A number of factors are probably in play, notably relative size. By the way, that old trick pf putting a marker stake or decoy at 40 yards or whatever can be useful but it's not foolproof. Accuracy decreases with the height of the bird over the marker. The formula is A square plus B square equals C square. A is the horizonal distance to the duck, B its height and C, the distance to the duck, is the hypotenuse of the resulting right triangle. For example if a bird is 40 yards out and 40 yards high, the range is about 56.5 yards, beyond the ability of most hunters. As a wise old hunter once said, "You can kill em high and you can kill em wide but you can't kill em high and wide."