How much weight you carry will effect where the armpit crease is located at. I heavy person will have an armpit crease that is wider than the point of shoulder. A lean person can have an armpit crease that is much narrower than the point of shoulder. The point we are looking for is pretty right in between. It is a historical measurement, so I can't explain why it was used. But the drafting techniques of the period rely on this measure, and I am using those same techniques to draft the Truly Victorian patterns.
In your case, narrow shoulders and large bust, you should be fine. If you end up falling off the chart, then you can choose a larger back width to get yourself back onto the size chart. You will need to trim the armhole edge of the front to end around your arm, and match the shoulder seam of the back. But other than that, you should not have too much trouble. If you do, feel free to email photos of your fitting, and I can help you find solutions.
why isn't the tape edge held perpendicular to the armpit crease? I've not used this form of measurement before so i thought I'd ask. Also, my daughter has a small shoulders, point to shoulder point(12") and a very large bust 47 DD am I going to have any difficulties not coverd here with fitting her? I have the polonaise pattern