People new to block making and patternmaking, especially those with little or no sewing experience, often don't understand the instructions for creating blocks. They simply just follow along step-by-step. It is often difficult or impossible to see if and when an assumption is being made within those instructions.
Even if you do sometimes understand an assumption - e.g. that the block will be made with a B-bust-cup, you sometimes don't understand that fully. For example: when you use your bust measurement to make the Bodice Block - how can it then be too small in the bust? Most cases it will be, if the instructions are assuming a B-bust cup. Even if you do realise that you have to do a Large Bust Adjustment to create a bigger dart, you still may be missing part of the picture; that the underarm width will be too big for you.
If you don't really understand the full ramifications of a B-bust-cup, you will make your B-cup block, then do a Large Bust Adjustment (LBA) from that block, and then.... be caught by the Large Bust Adjustment Gotcha.
I made a block to my measurements, and I was aware that the result would be a B-bust-cup. The toile was very ill-fitting. As well as being tight in the bust, it pulled from the armhole to the bust. It seemed big in the underarm area also - but that was being complicated by the pulling from the armhole to the bust.
It was obvious that there wasn't enough ease in the bust, and I needed a Large Bust Adjustment. The issue with the underarm and armhole, according to all the books I read, was due to the small bust dart... if I did a LBA, that would fix the problem. A few of my textbooks said something along the lines of: "if a loop forms in the armhole, that what is needed is a Large Bust Adjustment ".
Which I did.
Then it just became more wrong. It went from being too tight in the bust to enormous everywhere.
Of course, I thought I had made a mistake and increased the bust cup too much.... but that wasn't the problem.
There was a clue in one book; I can't remember what book and exactly what is said, but it was along the lines of: "if your upper body is smaller, and your bust is much larger in comparison, draft the block for a smaller size and add a large bust adjustment." But I couldn't understand what exactly is "a smaller size" - which of the measurements of the smaller size did I use? Those particular instructions I was using at the time didn't create the front and back separately, and I just didn't understand what "a smaller size" meant, in real terms.
A clue that didn't help me much then, but eventually led me to realise what creating a block with a B-bust cup really means.
The definition of a B-Bust-Cup is the Upper Bust is 2 inches smaller than the bust. A C-Bust-Cup would be 3 inches smaller in the Upper Bust, and a D-Bust-Cup 4 inches smaller (etc.)
So if you make a block using your Bust measurement, and the block is going to create a B-bust-cup. It is assuming the Upper Bust measurement of the B-cup (2 inches less than the bust), and the amount of ease in the underarm is worked out on that Upper Bust measurement. If you have a D-bust-cup, you will end up with an extra 2 inches ease on top of whatever amount of ease a B-cup will have. A DD-Cup will end up with 3 inches more ease than the B-cup.... etc.
The image titled Large Bust Adjustment Gotcha further up this page shows the extra ease I ended up with by creating a block for a B-bust-cup, then doing an LBA. The brown block is my correct block, the purple area is the unnecessary extra. Remember that the block is half the body, so that extra ease is on both sides of the body.
For a correctly fitting block, you need to do the bust shaping from the correct Upper Bust measurement.
Making the Block to your Upper Bust
The problem is that if you draft a block to your bust measurements, and you don't have a B-bust cup, you need to make adjustments to the Upper Bust (armhole area) as well as the bust.
I think it's more sensible to use the Upper Bust to draft your block. You can then create the correct bust cup dart following on from this. This is the way I do in my instructions. See my article: Why Upper Bust rather than Bust? for details of how this works.