I wear 7.5 - 8.5 in heels, 7-7.5 in flats.
I have a really super wide foot, this isn't usually an issue with flats which is why I can go with a slightly smaller size. 7.5 typically fits me the best length wise, but I usually have to go to 8 or 8.5 to squeeze into the shoes width wise. Sometimes I have to go higher, I have a pair of size 9 Pucci shoes, I really really liked them but couldn't squeeze into anything smaller. 2 pairs of shoes I took to a shoemaker for him to stretch, you can usually do it on soft non-patent leather.