Well, 50mm sounds about right for most smaller umbrellas like 45 inch and smaller. If you had say, a 60 inch umbrella, the 50mm setting might give a central hot spot,and a very strong fall-off at the edges.
Using the slip-on diffuser or any diffuser with a shoot-through umbrella doesn't make a lot of sense to me, since the diffuser tends to spread some light out,and prevents it from going through the umbrella. Shoot-through umbrellas can be used with the flash's zoom head set to a very tight,telephoto beam setting like 105mm; it changes the way the light hits and goes through the shoot-through umbrella. It's not necessarily "wrong", but it is different, to use a speedlight set at a tight beam spread or with the speedlight positioned really,really close to the umbrella, with the flash very close to the umbrella ribs.
(I want to say it clearly: I am not a fan of shoot-through umbrellas for use with larger studio lights or monolights. I learned with shoot-through umbrellas,and do NOT like their lighting quality, which with more powerful lights produces a huge amount of light scattered all around the entire shooting area, creating a lot of what is called ambient spill or ambient fill. I still think shoot-through umbrellas have some issues with light control,and work better for simply elevating the light levels upwardly,and providing soft, low-ratio, kind of flat, dull lighting that lacks much direction. For creating a main light source that is large, and for creating a low-level fill light from the spilled light kicked back off the umbrella, a shoot-through umbrella serves as a main light,and a fill light,all in one,and I think the effect looks very bad when done in small rooms. Others disagree, or have shot only shoot-thru brollies, and they might happen to like the look. As always. YMMV)
One place where I have seen some AWESOME results was when people used a speedlight flash with a reflecting umbrella, and the flash was fitted with a Sto-Fen type diffuser cap. That light is called "double diffused"; the Sto-Fen diffuses the light hitting the umbrella, from something like 25 inches away on the umbrella shaft, the light is quite diffused, and then it is reflected back to the subject,and the overall result is very soft light, for even a smallish umbrella like a 30-incher. This is a fairly new idea people have just begun doing, using an umbrella with speedlight + diffuser.
I like the Photoflex brand's RUT convertible reflecting umbrella for speedlight use, in both the 30 inch and the larger 42 inch model I think they call it...I call them "small" and "medium".