This tidbit hit me (some time ago) about picking the right size for a sewing pattern.
(Not the tidbit)
You should pick the size of the pattern based on your measurements NOT the ready-to-wear size you would normally choose.
Once you are ready to make adjustments to sew the pattern, you must consider the intended style of the pattern. This means that you should be looking at the ease and drape of the garment. Admittedly, this is very hard to get the whole intention from a flat image but, with practice, you will get better at mentally visualizing in 3D. If your cup size is larger than C then you should also think about an FBA or if the built-in ease is enough to give you the room you need without adjustments. Most ready-to-wear and sewing patterns (big four) are based on B or C cups.
(This is the tidbit)
In my opinion, the more fitted a garment is, the more important it is to pick the size based on the finished garment measurements especially if the style is for woven/non-stretch fabrics.
Now, with that said, it is up to you and the style you are going for when you decide to sew up a pattern. Sewing pattern instructions are the way to get the look as pictured/drawn. Since we all have very different topography, you're almost guaranteed to have to make some kind of adjustment if only to make the garment fit our personal style, even if your measurements match the pattern perfectly. For instance, you want to sew a basic tee and the pattern you have is basically a rectangle with holes (neck, arms, torso). You like it to be a bit slimmer not tight but, you wear a DDD cup with a much smaller waist. This means that below your bust apex there may be a lot of fabric blowing in the wind and you just want to take that in.
So to me, sewing patterns are the starting point or suggestions to get the intended look but, with your unique additions or adjustments for you! This is why you should never worry about getting the exact result as you see in the pattern image.
