Flat nipples flange sizing

When measuring flat nipples are you using the top of your nipple (narrowest part) or the bottom of your nipple (widest part) for flange sizing. My nipples measure 17mm at the top and 21mm at the bottom and struggling to figure out which size is best. My nipples are sore with all flanges and struggling with this pumping journey. I’m working on getting an appointment with a pumping specialist but in the meantime give me your tips. Thank you