Is it okay to cite a supplemental attachment when referencing a previously published technique or method? (advanced materials and surface science)

Question

The thorough answer to Roadmap and possible pitfalls for a first-time software author to publish a paper on a Python package? covers the issue nicely, and it's not yet clear if in my case this paper still needs to be published stand-alone. The reason is that the mathematics behind it will be sufficiently described in a supplemental materials document associated with a (hopefully forthcoming) publication in the general fields of advanced materials and surface science.

Now my question is if the supplemental section of a published paper in a well recognized journal will sufficient to use as a citation in subsequent papers e.g. "coefficients were extracted using the same analysis as previously described in X et al." (or something with less awkward cadence) where the bibliographic item will mention the supplemental section to that paper.

Question: Is this a generally acceptable way to do this, or are supplemental materials not considered to be "real" references, either because (perhaps, just guessing) they are not as thoroughly peer-reviewed as the main paper to which they are attached, or don't appear in the print version and so are not considered as accessible, or for reasons I can't yet imagine?

Answer 1

(Converting my old comment to an answer.)

Maybe there's variation in what is acceptable or commonplace, but (in physics) I've written and read citations like "following the method described in the Supplementary Information of Ref. [X]". To me this is not particularly different from referencing specific parts of a paper, such as a specific page, equation, theorem etc. The supplement is obviously less central to or integrated with a paper than such specific parts, but since it's usually published along the main paper, I consider it to be associated with it.