Butterick B5317 - The muslin

I have been working on the bodice muslin of my very first dress. It is all very exciting, my room is covered in red and cream threads and the ironing board has officially been hogged.
For the first attempt I cut a size 14 - not what I'd pick up in a shop, but I was following the measurements. I cut the outer pieces and the lining both out of the muslin fabric and slowly assembled them into something resembling a sports bra. I was inordinately pleased with the finished piece until I put it on. Sadly it was too big, so big in fact that I had to pop a pin in the middle of the back piece for the photo below.

 

However, I liked the shape, so I ripped it all apart (spreading even more fluff and thread everywhere) and trimmed the pattern down to a size 12. When I re-made it I didn't bother with the lining in the interests of time and so the pictures of the size 12 look much worse. 

The good news is that generally it's a better fit. The bad news is that the back is a bit of a disaster, I think the scoop is a bit low and wide. It looked much better in the 14 when I took some width out between the straps on the back, so I'm toying with altering the pattern. Is it a bit ambitious to make pattern alterations on your first time? 

The bit I found really awkward has actually been the cutting. This fabric has such a loose weave that it pulls apart and stretches making cutting exact shapes difficult, so I'm pretty sure everything is a bit wonky. However, apart from the neck ache I got from hunching over sewing away, I've really enjoyed it so far. I've got some fairly difficult bits to come - I'm imagining inserting a zipper is no walk in the park, but I shall keep you updated.