Basic bodice block drafting

1. Hi, I’m having trouble understanding how to draft a basic bodice block. If the measurement from my shoulder to waist is 40cm, but I plan to add a bust dart from the side seam that takes out 3cm, does that mean that I have to consider my shoulder to waist length as 43cm to conserve my original measurement?

Because the video that I’m following adds 3cm to the waist measurement to take the waist dart into account, but not the total length and I’m confused as to why.

2. Maybe a more fundamental question: how do I know how much I need to take out with darts in total? My understanding is that it’s the difference between the high bust and bust circumference.

For example, if my high bust measurement is 80, and bust is 84, the total dart lengths should equal 4cm… And I can distribute the 4cm wherever I want, 2cm on the bust dart, 2cm on the waist dart. Or 1cm on the waist dart and 3cm on the shoulder dart etc… is my understanding correct?

Thanks!