What size were you before bmx ?

There is no way to know what it means in actual cup size, and when it comes to TE's it does not really matter anyway. TE's can be underfilled, or overfilled, and implants can be greater than the fill size. TE's are hard and meant to expand tissues, so they will project more than implants will, for example. Your TE is establishing the footprint for your implant. 400ccs on you can be very different than 400ccs on someone else. Implant model/projection/width can also affect how a certain number of cc's looks and "sizes out to be" from one woman to the next.

The reason we don't talk cup size, is because there IS NO STANDARD CUP size. A 28D is not the same as a 34D. Many like to assume they "know" what a D cup looks like...but this is only relative to them and assumes they aren't in the 80% of women in wrong bra size anyway. It all depends on your ribcage measurement. All a cup size is is a a difference between ribcage and overbust. Every 1" difference is a cup (an A is 1", a B is 2", etc). So, a woman with a 32" ribcage and a 37" overbust is generally a DD. A woman with a 36" ribcage and a 41" overbust is also a DD. If the woman with a 32" ribcage wore a bra way too big for her at a 36 band, her cup size would be a C (but this is not recommended to do this!).

I for example, have a 29" ribcage measured snugly. My overbust measures
34.5". I round this to 35". I wear a 30-band. The
6" difference between my ribcage and overbust equates to an "E" cup in UK sizing (would be about a DDD
in US sizing, but US sizing is not as consistent as UK). I was also a 30E BEFORE my surgery, but my breasts looked different. I had 371g removed from one side and 446g (or thereabouts) from other side. My measurements are still the same though I have less projection and "appear" smaller - the width and firmness of the implants compared to my natural tissue has me with the same overbust as before. It would have been useless to talk cup size with my PS, as what would an "E" cup mean to him? Or a "D" cup for that matter? I told him what I liked about size of my natural breasts, and what I did not, and gave him ideas of how I hoped to be relative to my natural size.

So, no one here can tell you what YOU need to be a "D" cup, and that is
why cup-sizes aren't talked about. It is best to communicate to your PS what your expectations are without talking about cup size as not even most PS's understand cup size either. Take in photos of what you like. Tell him as you are doing fills what you feel about the size you are at that time, compare what you want to what you had before.