I'm sure the inconsistency of retailers' random vanity sizing has been done to death
But I tried this web app, which crunches different brands' size guides to recommend which size you should wear based on your own measurements
http://sizes.darkgreener.com//_
Wish this was updated with more brands, as this is the sort of thing that would be so so useful for online shopping! I'd be really interested to see how H&M and Hush stack up, as I find their sizing comes up particularly small / large respectively. eg most shops have me at a 6 - but I'm simultaneously a size 4 in a designer brand (size 2 US / size 6 UK leather leggings from Net a Porter were too big, had to get the size 0 US) yet also a 10 in H&M (trying on denim shorts I couldn't even zip up the 8). Madness!
Just posting the link here FYI in case anyone hasn't seen if but may find it useful