Honestly I found it so weird that people of a similar build to me get to size d/dd with 300-350cc.
I’m 6 weeks post-op and had 325cc under, and I’m barely a C cup ( I can fill padded C bras, but not non-padded ones).
Regarding dropping and fluffing, so far mine haven’t changed the size! I have a target bra that I’ve kept trying on once a week since week 1, and I haven’t noticed any change in the size.
I was also anxious about ‘too big boobs’ before surgery as I thought a size C was ideal, but think if you more prefer D over C, go for 400, especially if you go under muscle.