Hmm, I find Chloé flats difficult to buy online - size wise that is. Better if you can try them on.
I have two pairs of chloé moccasins. I have a pair in size 40 and a pair in size 41. The leather is really soft and the ones in size 40 have gotten almost to big after I have been using them all last summer while the ones in Size 41 fits me perfectly even though I have been using them just as much. And I'm usually a size 40 - almost never at size 41?!
So to sum up

my experience is that because the leather is so soft it is hard to tell whether you should go up or down in size.
If your looking for the flats moccasins with the C-buckle net-a-porter got them in 4 different colours as far as I can remember - taupe with gold, chocolate with gold, metallic taupe with silver and green/blue with enabled.
Unfortunately they are not on sale though - but I can highly recommend a pair.
Good luck with your search!