I wear a 9 in Nike and 8.5 in Adidas and generally anything size 8.5-9.5 fits. Size 8 qasas fit snug but there's still definitely room in the toe box to where I could fit in a 7.5 probably, but joints would be way too narrow/tight. Alternatively my retro boost are size 9 and there's for sure a bunch of room in the toe box enough to the point where I'm going to buy another pair full size down, but I still wear them and like how they fit.
Really just depends on how you like your shoes to fit but definitely at least .5 down