Well, not if you’ll be wearing a bra with the blouse.

Most likely no. Your bust is lifted when you are in a bra, and unless you are an A cup, your measure across the bust will be different without a bra.

I used to measure women for bras and we measured them in their ill-fitting bras, because it’s more accurate than no bra.

Probably. I hear most women aren’t wearing a bra that fits properly. But it may not matter at all…

No. You want measurements that fit where your breasts are with your bra on.