Vivobarefoot Primus Lite III sizing

I know that there're a lot of posts like this one, so my apologies in advance for that. I've read a bunch of them, and until now it's still not extremely clear how to think about this in my case.
I'm often a size 42,5, but I had to go for 41 in the Vivo. They are my first pair of barefoot shoes, so I'm still learning how they should feel like.

The 41 as it is possible to see in the photos, is fine, but I feel that the heel is a bit too loose. Aside of that they feel great. Tried as well the 40, but the upper pressed on my toes as it was too low. With the 41, I have no issues at all with that.

Based on your experience, can someone give some advice on the fit? As I said they feel ok, with the only thing I'm not sure of, being the heel and the length. It's not as if it jumps up and down, but there is some space that I don't have in normal shoes.