I ran your spherical 5.5" mirror through Tex and into Aperture. Aperture says that the transfrer function of your mirror is the same as a perfect mirror 10% smaller than yours. It also shows that 91% of the focused light will be in the diffraction disk.
The Tex program says your mirror is about 1/9 wave.
What does all that mean in terms of seeing? I don't know.
If it was me I would try to figure it. If you try long enough you will be able to get it. However you are very close to 1/10 wave already and Suiter says that there is enough error in reading the Foucault test to not beleive any rating over 1/10.
Dale Eason