going down 1 size left me with 1/2" of space or more. It was at the end of the day top, so my feet were already swollen to the size they'd be at the end of a work day. I have a pair of Rain boots in 8UK bit I can wear thicker socks with those that no one will see.
Well then I guess you have to do some trial & error by going down size & half or the rain last is not for you bud. You are the one who can make that decision.