No you don't need a different scale for each lens. Read up on the physics of it, it's determined by extension, and hence the same scale works for every lens.

I'm sure you can in some cases do it fine by loupe, but why bother performing what is in essence a purely mechanical mathematical task manually when a scale is faster, more reliable and repeatable? It's basically converting a focus spread based on extension into the sharpest aperture to stop down to. There needn't be any human decision making there. Likewise we could probably all make a decent guess at exposure just by eye but most of us tend to use a light meter. Same idea.