Trouble Determining the Right Dress Size

First off, I used a web-based Dress Size calculator to figure out my dress size. I measured all the necessary parts just seconds before entering the data. The outcome was roughly a size 18. But recently, I tried on my girlfriend’s nurse uniform (it’s the first post in the CD picture thread for anyone who cares), and it fits perfectly (as you can see:P). It turns out to be a size 12 (I believe). So I headed over to a nearby shop to buy some items for both of us. I decided playing it safe by opting for larger sizes was the right move. Unfortunately, they are just too loose. The corset “fits” but doesn’t feel like one; it wraps around my body without providing that necessary squeeze. Sadly, I couldn’t return or exchange them because I lost the receipt. The other piece is a lace-style teddy, also slightly oversized and labeled L/XL. It was marked as non-returnable when bought. So here I am, puzzled and a bit frustrated about what to do next. I spent just over £55 on two items that didn’t fit right. My question for the group: Is there an easy way to get the sizing correct? Ideally without buying first and trying later (which makes me lose receipts again lol). Note: I’m in the UK, so sticking with UK sizes would be helpful. If needed, I can share my measurements even though they were taken a while back.