I have a high instep too. I usually wear 8.5 /9 UK. My CJ Northcote boot ( 348 last) in a 9uk is too big for me;I should have taken 8.uk. With Vass I wear 42.5. so my Rumanian Racing Green Osloer boots are 42,5 EU and still a bit loose. Just two examples.
The shade is delicious but the boot is even more.