I posted this over at the Curvy Sewing Collective's Facebook group, and thought I might as well stick it on here too.
A major problem that people have who are not a B/C cup is that patterns use full bust measurements to judge how big a bodice you should make. The bodice, at every size, is proportioned for a B/C cup lady, so if you pick the size that fits you and you have a larger or smaller bust then it will end up the wrong size as it is based on a different shape. The easiest way to get the right proportions thorough your shoulders, neck and back is to use your high bust measurement.
The diagram above shows two ladies with the same size bodies but different size boobs. Both ladies need a pattern with the same size shoulders, back and upper chest, but the E cup lady can't work out what size that is from her full bust as it will all come out too big, as the bigger bust size is just larger all over.
On the E cup diagram the lower red line is showing the measurement to the full bust of the B/C cup lady, so that you can see it is basically the same as the measurement of the high bust (the top red line) of the E cup lady. Using this measurement, the E cup lady can start with the size that fits her upper body apart from her boobs, then just do a FBA to add in extra boob space at the front.
Whilst this shows the general idea! (I hope), when measuring yourself, you don't have to have the tape on a crazy diagonal as in the picture, as your back gets a bit wider as you go up and it's wider where your arms attach, which compensates. Just measure where it is comfortable to do so, it shouldn't make that much difference, and should certainly be a better fit that using your full bust!
This should also work if you are smaller busted, you just do a Small Bust Adjustment afterwards instead.