Q. When using a letter + noun, should there always be a hyphen, never be a hyphen, or something in between? Specifically, something like “k value” or “k curve”?
A. According to section 5.4.1.2 in the ninth edition of The CSE Manual (published in 2024 by the Council of Science Editors in cooperation with the University of Chicago Press), letters that are used in scientific terms as modifiers can remain unhyphenated except when the term itself is a modifier before a noun: “S curve” but “S-curve characteristics.” You’ll also find “S curve” as an example in CMOS, under paragraph 7.70.
You’d also want a hyphen when the letter is paired with a participle before another noun, as in “k-shaped curve,” an expression that works better with a capital K, whose diagonal strokes more clearly suggest the idea of a graph with diverging values: “K-shaped curve.”
There are some exceptions. For example, we would refer to the “x-axis” (following Merriam-Webster—except for the italics, which aren’t usually shown in dictionary headwords; we italicize the label because it derives from the mathematical variable x). Another exception is “x-ray,” which is hyphenated as a matter of established usage and often spelled with a capital “X” (but without italics, maybe because the letter simply meant that the type of radiation was a mystery when it was first described, as suggested under “Word Origin” in Merriam-Webster’s entry for the term and by the term’s etymology in the OED).