@CreativeSheep I'm afraid you're kind of on your own with the math, but a "standard" is something like 10px/cm, which equals roughly 1K per meter. I use Maya; I punch in a couple numbers and know exactly what my texel density is. 🙃
@CreativeSheep What software are you using? What happens to you UVs when you change those numbers? Can't really help without more info. As @Leinad pointed out, you need to know what your texture resolution is, as texel density is a ratio (pixels/unit).