Flange inserts measurements

Im a first time mom-to-be. From what i see pumps usually comes in bigger sizes like 24/28mm however i have a smaller nipple and need to buy the flange inserts. However im not exactly sure how to calculate the right flange size. I know theres a ruler for it but I googled and a some say add 4mm to your nipple measurement some say 2mm. Anyone knows? And is it best to buy before or after delivery? #advicepls #firstbaby