I need to make a watch case screw without having the screw. I tried to follow Fried's book, bench practices. I understand how to determine the proper diameter by inserting a brass dowel into the existing case hole then add 1/4 the thickness for the threads. But I don't understand how you determine which thread size to use. Can't a screw have a variety of thread "densities" or so many threads per mm in length. I don't understand how you determine that if you don't have a screw to measure it on?