I started the same thread a few weeks ago.
My heaviest was 18:4, and I was in 20/22 tops, trousers a whole different kettle of fish, they ranged between 18-24 depending on the style and where from.
I'm now 16:13, I haven't been shopping cos majority of my tops have shrunk in the wash over time lol. My size 18trousers I can get on without undoing them, and have to pull them up regularly, but I know a 16 wouldnt fit me now, maybe as I lose a few more lbs in the 16's I'll be able to fit into them.
I carry my weight around my belly, I always have, so tops are different for me. Every1 is different. If I could fit into say a size 16 top and feel comfortable then I'd say I was a 16. I think it's about what size is comfortable for you, does the waist band cut in, can you sit down without feeling like your being sliced in half.
I remember being between 10-12st 5yrs ago wearing 10/12. My goal is to feel comfortable in a size 14 but my weight goal is 11:4, so I could be a 12, only time will tell!