There are a number of “rules of thumb” for determining the best screw diameter to use for a particular mold. However, the 2D to 3D method recommended by many materials manufacturers is a more accurate approach you can use with confidence.
This method may not be as widely used, not because it is not a good choice, but because it can be difficult to work through.
Since I spend a good amount of time helping processors size equipment, I look for tools to help me that are quick, accurate, and easy to use. I developed a spreadsheet for screw sizing that I have been using myself for a couple of years now. I handed it out to some processors, with good feedback.
You can have a copy of this spreadsheet for your own use.