I know the feeling. I got lucky with fins but perhaps it was not entirely luck, over the years I've learnt a few things about shoe(/fin) sizing:
1. The
foot measuring devices used in shoes shops are consistent but wrong, they consistently measure my feet 2 half sizes smaller than I can actually wear.
2.
Manufacturers: Sizes vary a lot from manufacturer to manufacturer - especially on width. For example Nike shoes are narrow, La Sportiva boots & shoes are narrow, New Balance & HiTech are wider. Boreal, Spain actually make wider shoes for the British market - bless them
3.
Sizes: American sizes cannot be relied on, especially around the size 11/12/... area, it seems manufacturers went their own way on larger sizes. British sizing is a little better but still unreliable. Surprisingly to me, the EU sizes seem to work and are pretty consistent (but see #2 above!) - my feet are: USA 11-12.5, UK 9.5-11 and EU 45.
4.
Conversion Tables comparing different sizes - typically UK v. USA v. EU differ considerably and, in my experience, most of them are very badly wrong.
So the best advice I can give is,
find the best EU size for your feet - through experience
not by measuring devices - and use that as your guide. Get the correct size or slightly bigger, as you can pad with thicker socks or perhaps even insoles if too big;
do not buy footwear small there is almost nothing that can be done for footwear that is too small!*
*Anti-clever dick clause: there are a few (very few) cases when small, tight-fighting footwear is desirable/required - high performance rock-climbing shoes/slippers are the only one I can think of (perhaps ballet shoes?). It is possible (but not recommended) to stretch the leather in some leather shoes.
For all the complications mentioned in 1-4 above, manufacturers/seller sometimes provide a foot length & width in cm - in the 2 cases when I have used that as my guide, it worked remarkably well.
