If I understand your assumption correctly, it won't work. I don't see the connection between texture count and surface coverage...also set texture according photo width...this could work teoretically only in very specific case and manually unwrapped mesh.
Before texture creating process you don't know how MS will unwrap your mesh(how many cuts, how big chunks "UV islands" will be in texture UV space, ...what will be the texel density)...so the only option is to create one test texture 4k/8k and make visual guess how many times is the texture on your model more or less sharp. And even after that guess, if you set the amount of textures, MS will unwrap your model differently, because the mesh will be unwrapped not into one(like before) but several UV square spaces....so different count/size/rotation/position/margins/... of UV islands in UV spaces.
The rest guidance(how to unwrapp in external app and calculate texel/pixel density) I mentioned is in those other posts and topic, so for now I don't have nothing better to advice
. From my perspective if we don't know the texel mesh density value, any calculation will be not correct. If I missed something, len me know.