The pollster must be rated at-least 2.5 stars on 538's pollster ratings
https://projects.fivethirtyeight.com/pollster-ratings/

and obviously I have to be made aware of it (and be able to access the full detailed results needed for above calculation). I might miss a just a one off poll where this barely happens so please comment it here if you want a guarantee that I am aware of it.

Example: In this Gallup poll taken at the end of his term after 71% of those whom identified themselves as being conservative approved of him
https://news.gallup.com/file/poll/328649/210118TrumpFinal.pdf
which
This is the lowest approval rating Gallup has ever recorded for him and took place right after the insurrection. Less then 50% approval among this group would still require a 30% drop from this figure.