So are you suggesting to try and adjust the pressure of the retaining ring?
Adjusting the pressure on the retaining ring will make no difference.
If you can not see a very clear strong Newton ring pattern in the lens, then your lens is fitted
with a spacer ring, and adjusting the pressure on the retaining ring will make no difference to the S.A.
With my scope I replaced the spacer ring with thinner spacers, the star test shows a better ring pattern each
side of focus, but it still has slight under-correction, but better than when I first received the scope.
There is only so much that you can do. While being an achromat, my scope still shows plenty of Planetary detail.
Here is a drawing I made of Mars, each drawing was made 24 hours apart but in the same seeing conditions,
one was made with my 120mm Achromat and the other with my Takahashi TSA-102.