With all due respect, the relationship between a bullet and the proper twist rate has little to do with bullet weight.
The Greenhill Formula for calculating twist rates does not require any weight input, though there is input for specific gravity.
Yes, but bullet length and bullet weight go hand in hand. In order to make a bullet heavier, you have to make it longer, so twist rates coupled to bullet weight is a pretty good rule of thumb, if not perfect.
If you are casting bullets out of aluminum or 24K gold instead of lead, this does not apply.