The Muslin That Wasn't

Before cutting into the "good fabric" I wanted to make a muslin for Megan's t-shirt. I used the same blue sheet that I used for my t shirt. I think I cut a straight 14 for her, as a starting point. Her bust measurement fits a 14, but she does not have a B cup. So does that mean I should be making a 16 and do a SBA? I figured on a t shirt it doesn't matter much, but the neckline is a little low (I think I did raise it for her because that's one of her issues with t-shirts and gym class). There is gaping at the neck which could be because I didn't stretch the neckband enough, or it could be sizing. I did a cheater SMA on the one I made from the good fabric. 

 There is some pooling at the back. She's about my height, 5'1". I don't want to take in the CB seam but would taking it in just a bit  help?

The neckline looks like it fits, but the band is a smidge floppy, and there is the gaping when she bends over issue. I think it could be narrower on her shoulders, combined with a tighter band.
I hemmed with a simple zig zag.
She says it's SOOOO soft and instantly loved it. But then she started wearing it less because of the neck gaping. I think it's time to re-visit this before school starts so she can have a couple of comfortable gym shirts. Which will mean she will instantly grow a bust and need a whole new pattern.