I now publish some measurements.
It looks like the springs have made the mirror holder very stable.
First of all -
see first picture -
just the mirror cell when placed flat on a table without the mirror.
First picture – the mirror was surprisingly central when checked with the mirror lying flat in its holder on the bench.
The outside metal ring fits into the carbon fibre OTA tube with a proper groove.
Using a set square and my digital vernier I was able to make accurate and precise measurements.
The result was that 3 measurements agreed to better than 0.7 mm.
So in this state the mirror is “exactly” centralised in the OTA tube.
I am happy about that.
Next -
see 2nd picture.
with the telescope placed so the mirror end was over the edge of my table
so I could use my digital vernier in all rotations -
I measured between the central marked cross of the mirror and
the outside of the metal ring -
with the springs labelled A, B, C - in line with the springs.
I did it twice so 2 full 360 degree rotations
and recorded the results.
Note:
for my laser - after adjusting -
the error is less than 5mm at 4.3 meter target distance which
for my 1016 mm telescope will represent less than a 1.25mm error at the mirror with different rotations in the focuser.
My mirror centralising error is at most 1.7 mm.
The mirror will never be horizontal as per the test -
that is an extreme case.
It won't usually be below 45 degrees so the actual error
will be less than 1.7 mm in actual use.
Part of that could be my measurement error.
Therefore I think I can say that my strong springs have worked. 
Any error with that laser in different positions is not coming from the mirror holder movement.
Maybe there is a movement of the secondary or the focuser
which caused the laser spot to move slightly upon rotation?
see my post:
24-02-2023, 01:41 AM
Any thoughts would be appreciated.
cheers
Allan
Edit -
correction factor.
I measured between the central marked cross of the mirror and
the outside of the metal ring -
that is at an angle so the actual measurement is less.
The depth of the mirror from the back of the metal ring is 84.00 mm.
Therefore a measurement of 163.0 mm on the hypotenuse is at an angle of Sin 84/163 = 31.0 degrees.
The actual distance of the center of the mirror to the outside of the ring is Cos 31.0 x 163 = 139.7 mm.
The correction factor is 139.7/ 163.0 = 0.857 = 85.7 %.
So a stated maximum error above of 1.7 mm is actually 1.7 x 0.857 = 1.5 mm.