Confused on sizing???

I measure 15mm. I’ve been using 17mm plastic flanges with my spectra and they don’t give me any issues. I recently wanted to try the legendairy inserts for my eufy and I heard to stay the same size with wearables so the nipples stay in place. It felt fine and it was pretty comfortable. But in just confused in general. Should I be sizing up, down or not at all? Is there different sizing with different materials like silicone vs plastic?