Yes, exact quantificaition can be difficult, but obvious problems are easily detected.
The problem I had was that without the obstruction, color aberration in a refractor makes the ring brightness hard to judge. I have tried filters, but still because the peak intensities can shift due to spherochromatism, it becomes difficult to make inside and outside look very close, and the eye is a horrible discriminator of brightness differences and what one observer sees as a larger difference, the other may say is a subtle difference, to there is no easily applied uniform standard.
The obstruction allows enough sensitivity reduction to counter some of the other issues but higher order sperical aberration that has no impact on the in focus image can still make the balance between the ring brigtness appear different.
This is why I suggest trying to ignore the appearance of the rings. The SA test really only depends on the secondary shadow size ratio between in and out of focus, and will show a larger error quite easily (1/6th wave), but when you get into tiny differences, it can be more difficult to estimate small errors.
And it bears repeanting. I do not know if this particular scope has been aspherized to reduce spherochromastism, but at this aperture and this speed, there is going to be some present, and you cannot dismiss the possibility that this is being presented in the star tests..
A question.. Do you know the design polychromatic Strehl? Not talking about the finished instruments fabrication quality but rather the absolute polychromatic Strehl for the glasses, aperture, and focal ratio used. I would be curious to know exactly how much light would not go into the Airy Disk even for a perfect instrument.