Wise choice. Bigger is not better. One of them aims of sizing during consultation is to ascertain if the size that you like can look natural.
For a DIY technique to decide how much volume is required to get you to your bra size of your choice :
- Buy the bra that you want to fit into
- Fill a plastic bag with enough fluid to fill the bra. Rice may be less messy (and can put left overs to good use too)
- Measure volume of fluid or weight of rice
I am afraid that does not answer the question of whether it looks natural. That's probably best answered after a consult. But at least you have done some ground work.
Hope this is useful!