At least you don't want to make the secondary holder so tight that you risk creating some kind of tension/pinching in the corrector plate (but you knew that already, I'm sure).
I just wanted to add one disclaimer. When I wrote up there that centering of the secondary in the hole is not that critical, please understand it in the context. I'm definitely not advocating anyone to mess up the factory centering just for the heck of it. At least according to the folklore, some SCT secondaries have been touched up at the factory to correct optical problems in other elements, and may therefore not be fully spherical anymore. What are one's odds to have an intentionally non-spherical secondary, I have no idea.
But when one already has a secondary that has become loose in the corrector (seems to happen a lot, and has happened to yours truly), you kinda have nothing to lose (sorry for the pun), and the main point in fixing the issue is not how to hyper-accurately re-center the secondary in the hole of the corrector.
If it was really critical, e.g. every sorbothane gasket sold by Starizona would mean an SCT lost, because once the secondary holder has been detached, there's no way for an end user to re-attach it in the exactly same spot it came from the factory. And the way these scopes are built also strongly suggests that centering of the secondary/corrector system as a whole is more important (or at least sufficient - there are built adjustments for that, unlike for secondary holder centering). This article includes excellent discussion about the subject, with instructions to center the corrector plate and secondary holder together, if the whole system is obviously decentered. And the article comes with appropriate disclaimers as well, naturally.
Edited by Axunator, 01 January 2020 - 02:43 PM.