Its good to be sure when choosing a new frame especially one thats so expensive but I think you are overthinking it with all this stack and reach stuff.

Standover: 819 mm = 32.2 inches.
Whats your inseam?
Whats the standover height of your current frame?
Top tube length 550 mm
How long is the top tube on your current frame?
I asked a friend of mine what the seat tube length on his 54 bk was and he said about 57.5 center to top.
How long is the seat tube center to top on your current frame?
If your current frame fits this should be like common sense choosing a BK size.