I sized down .5 on the loafer in F last compared to my oxfords. However, this was in Shell, so not sure about Calf (oddly, I take .5 smaller in Shell than Calf on F last).
**My sizing is a bit different, but seems consistent with your loose fitting rain shoes.
6.5 UK Rain = 40.5 U last for me.
@beargonefishing Thanks for asking.
Only the navy pair on the left and I was very cautious I admit; however on the brown pair I used neutral polish which should work for a while.