I figured out my sizing in the lasts that work for my foot, as well. I can always order more in the future
I really like the U last and the P2. They obviously fit rather differently but are quite good for my feet.
I haven't tried a tall boot in F, but having just tried on an F last chukka, my inclination would be to go for the same size in a bal boot. 43 F works well in an Oxford and in a chukka - I won't mess with success. Besides, I am given to understand that Vass makes their tall boots very slightly roomier overall in a given size and last.