I go TTS on the marlow ptb and marlow wt and Lindrick boot. I wear a medium thickness sock. I probably could go 1/2 size down on all three models but I would be restricted to a very thin sock. I have tried on the Gianni. I found it considerably bigger than those three models. I would need to go 1/2 size down (or perhaps full size down) on that model. But frankly it just didn't fit my foot very well so I took a pass on it.
Interesting. So for the Gianni you would suggest sizing an entire size down from your US Brannock? Say I'm a 13D Brannock, you would suggest a 12D Gianni?
Hard to say. I can only tell you my experience, which was that the Gianni TTS for me was waaaaay big, and I have my doubts that a half size down would have done the trick. But I was only able to try on the one that was TTS and no others.