man i couldnt get simpson last to work. i sized down 1/2 from my US size and it was too long. when i sized 1 down (same with Robert) it was too tight. Youre right about Rain last having a large instep (its also a touch wider than regular width). i ended up sizing down 1.5 sizes from my US size so I take a UK10.5.
my carmina sizes:
All boots or shell unless otherwise stated
Inca UK11 (calf double monk)