I made a DIY softbox for my flash, using cardboard boxes for the box itself, black card stock for the outside look, aluminum foil lining inside for the light, and nylon cover for the diffuser.

How can I check it's quality? What's the best way to check? I read something about standing 3 metres away, and getting the settings down dark enough so when you use the flash, there'll be a neutral grey sort of colour - am I on the right track?

Reason I ask is, using flash without it, yes, hard shadows. Using flash with it, I definitely get much softer and pleasing shadows (I'll have to get sample pictures up here next time as I deleted my test ones already but I'm happy with the quality from my tests).
But to make sure it's good, I compared it to the built-in diffuser screen on my flash, and I get the same results, or at least, very close results (only difference the softbox makes the lighter just a little cooler, and the light is just that little bit dimmer). Trying test shots on walls (so I don't know if I did it correct or not, as mentioned above), the screen and softbox look very much the same in terms of spreading the light and making it softer (again, screen diffuser just ever so lightly more brighter and warmer).

It's dimensions are 26cm length x 20cm height x 3.7cm nylon from the flash

So, in asking this, are the results good? That is, is it quality enough to use in real world?
Or are the diffuser screens on the flash generally good enough for use too?
Or if in a real world situation, there would be a noticeable difference?
Or is it because the box isn't very deep, quite shallow (3.7cm depth only), is that why the light doesn't really spread more or look too different from the screen diffuser? Does it need to be deeper?

Images taken from phone:
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I'll be sewing in the edges of the nylon to make it look nice when I get a chance.