What measurements do you use as a guide? I find measuring the depth of a shirt I own that has the kind of fit I want for my project is the most accurate.
When I get just above the point where I want the sides to join I do a few increase rows (every other row) usually only 2 (increasing only 4 stitches on each side), then I use a backward loop cast on to add the rest of the stitches all at once. The more increase rows you do before you cast on, the deeper the armhole will be so don't do too many. The backward loop stitches are going to seem lose, but once you start knitting down they even out and you can fix it when you cast on for your armholes.
Hope this helps.
ETA also I use my high bust measurement rather than my full bust. I don't think it matters much for people with an a or b cup size, but if you are bigger than a c you will get a better fit if you use your high bust measurement and then add short rows for your full bust.