What size to buy for a cleavage bra? Not looking for a bra that fits

The common advice I see on here is to go 2-3 cup sizes down, but do I keep my proper band size or should I size up in the band? My normal size is a 28C/30B so should I get a 30A or a 32AA? I'm looking to buy a la senza equivalent of the bombshell bra and was unsure if I'm supposed to size up in the band for a cleavage bra or keep my same band size.

Maybe neither of these is the answer and I need to find a push up in 28A?

Sticky bras work but I'm allergic to adhesive so it comes at the cost of painful welting rashes that I'd like to avoid.

My measurements as of today: 29, 28, 27.25, 30, 32.5, 29. The calculator recommends 28D but I already know it's a liar and overestimates me consistently by a cup or 2. (Conical shape and soft tissue).