One 45 inch reflected umbrella, fabric at 40 inches (a distance not exceeding the umbrella diameter).

Almost as good, and 18 inches is not always practical. The greater distance fills less well than if it were closer, the shadows are slightly darker, but the difference is subtle. The umbrella shaft does prevent it being as close as shoot-through can be, but reflected is a wider light for larger subjects. Close as possible still applies, and a distance not exceeding the "diameter" is a typical close distance for portraits (sort of a guarantee of softness). The sitting subject typically can reach out and touch the light stand pole of the main light (not the fill however, it has to be more back with the camera to be out the way of the lens).

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Here is another case - a comparison of shoot-through and reflected umbrellas (same umbrella, at same distance). Note that this "1" and "2" are very different than the original 1..6 numbering. An afterthought, and here, 1 is reflected and 2 is shoot-through, at same distance from umbrella fabric.

If both are metered at the same distance from the fabric, the reflected is one stop brighter than the shoot-through, which means 2/3 of the light is reflected, and 1/3 goes through. The black cover prevents that 1/3 from being any problem for reflected, but nothing prevents the 2/3 from being a problem for shoot-through (spill reflecting all over the room). However, shoot-though is normally only used very close to the subject, like 12 or 18 inches (closer than reflected is able to do is the entire point), so inverse square law says a 6 or 10 foot path from wall reflections will be way down, relatively.

Some say that a shoot-through is softer than reflected. I have tried really really hard to verify this, but I always get the opposite result. I believe it to be the wrong answer. It seems obvious. Shoot-through suffers from the edge of the umbrella being wrapped back, that portion not being aimed at any subject, and therefore the effective diameter is obviously smaller, quite a bit smaller relative to the light hitting the subject. Reflected is simply much larger (effectively), and the outer rim redirects light back to the subject, instead of away from the subject. Simply look at the setup and this is quite clear. Smaller is definitely not softer.

Yes, shoot-through obviously can be placed closer, and closer certainly can be larger and softer, which is why we are interested in it. Shoot-through can be placed closer than a reflected umbrella, and sure, it ought to be softer there, which is why it is done. But it is not softer when at the same distance to allow any comparison of the umbrella modes.

Try this.. Get a couple of eggs from the refrigerator. Handpick a couple with rougher texture to their shells. Put them in a saucer. Fill the frame with that dish, and the shadow under it. Three feet is about about as close I can get the reflected umbrella, so set it up carefully measured at 3 feet to the fabric. Also take another picture, same picture, but carefully measured to be three feet from the shoot-through fabric. I agree both at 3 feet is not actually equal, but not sure how else to do it.

Reflected umbrella at 36 inches to fabric - flatter - more wrap around on eggs - surface flaws less visible

Shoot-through umbrella at 36 inches to fabric - more contrast - less wrap around on eggs - surface flaws more visible

First picture (reflected) is obviously the more soft light. Has to be, it is obviously larger at the same distance. Brighter too, and vastly less spill. To me, shoot-through is a pretty special case, and I consider the reflected umbrella the better choice in every case where it can be used.

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A test situation below: Metered at 5 feet carefully measured from umbrella fabric at apex, using makeshift plumb-bob. ISO 200. Full power SB-24.

B&H Impact 32" umbrella.
Reflected f8 + 8/10 stop
Shoot-through f/5.6 + 8/10 (1.0 stop less)

Smith Victor 45" umbrella
Reflected f/8 + 9/10
Shoot-through f/8 + 1/10 (0.8 stop less)

Photogenic Eclipse 60" umbrella
Reflected f.8 + 9/10
Shoot-through f/8 + 2/10 (0.7 stop less)

There is only about half of this difference at 10 feet, but who would use shoot-through at 10 feet? The worst part is all that rear spill from shoot-through, the majority of the light is scattered all over the room (instead of directly aimed at subject) - but frankly, which is no particular issue when used very close, closer than reflected could be used (as intended).

Reflected is stronger, larger, softer, and much better behaved. I would choose reflected in every opportunity when distance allows reflected to be used.