i just go into reloading , i am loading 308 which i just bought the comp die set full resizing dies i also order the gold medal dies for my 300 ultra mag , it has where you can set the neck tension , so it tells me to measue a loaded bullet and the add 2 to 3 thousands and order that size then subtrac 2 or 3 thousands and order that neck sizing die , does this sound correct . and will this help with acurracy ?should i just use a factory round ?for my measurement