Slopers, or, Why you should make a mock up

I'm preparing another costume (I know - too many. but there's no such thing as too much cosplay!). The bodice is a strange shape: not quite a princess seam; so I scrapped my original plans and drafted a sloper. I am so glad that I made a mockup, because this sloper is abysmal. It gapes weirdly at the armpit, and I don't know how to fix it. Oh well, back the the drawing board.

My next idea is to take the original bodice pattern that I had in mind, draft out the princess seam (Dawn Pages has a great tutorial for that http://www.reddawn.net/costume/princess.htm) and draw in my own seam lines. It would make the front of the dress much more accurate. I'm thinking of altering this pattern (http://www.burdastyle.com/pattern_store/patterns/high-collar-short-wedding-dress-32011) All I need to do is to make that centre piece a little bit narrower.