I have done similar mirror experiments after having read your previous postings. Not to take anthing away from your insightful and meticulous trials, is it not enough to just place mirrors at the 12, 9, 6, and 3 o'clock positions and one in the center of the dish. If they all focus in a tight cluster onto the LNB surface then is it safe to assume that the rest of the dish surfaces are also in proper alignment?
Good question. Five mirrors which you used can locate the focal spot (useful, if the LNBF bar was missing or damaged). For positive verification of correctness of paraboloid, even 19 mirrors (which I used) would not be enough. This could be seen on the following photo. About 2-3 years ago I was experimenting with Seesat dish.
When it got dark, I hauled out a 500 W halogen lamp.
And this can also be confirmation of Kodaz observation : when light falls at sharp angle, irregularities become visible.
The dish's bracket was made of sheet metal much thicker than the dish's "bowl". The bracket was NOT shaped properly, and when the four carriage bolts were tightened, the bracket "won", thus distorting the dish's surface.
In daylight, not very noticeable.
Obviously, the four distorted areas do not return the waves to the LNBF. And, small mirrors, if not placed on these areas, would not "report" that something is wrong.