I have to go up in U. FWIW for me a 43 in F fits great, 43 in P2 is fine but a little roomy, 43.5 in U is great. When I first tried U I had ordered a 43 from NMWA and it was way too tight in the toes (and maybe overall volume too, I can't remember exactly). Sizing up to 43.5 worked for me. Obviously there isn't a consensus on this, but that's what I wear.
And to add to the Vass sizing confusion, I wear 43 in U, F and K but go down to 42.5 for P2.
Fred, let me know how those 43 F last chukkas fit.