Willamette Shirt in Cotton Voile

So... several months ago I said on Instagram that I would complete the Summer of Basics challenge: to sew or knit or make three self-defined "basic" garments during June, July, and August. I only made two of the pieces I had intended to make, and here's the first one:

This is my first Willamette shirt, designed by Hey June Patterns. I used kind of a wild print for something considered a "basic", but I think it could go with almost any neutral pants, shorts, or skirt. It could be super casual, with a Hawaiian shirt vibe, or even business casual, tucked into the right skirt or pants. It can cover a lot of bases, so I consider it a basic.

The fabric is an organic cotton voile from Cloud 9 fabrics. The design is called Yucca Paloverde. It's a little sheer, so I didn't wear this without a tank top underneath. In the summer, I don't like to have to layer multiple shirts, so I didn't wear this as much as I thought I would. But now that the temps have cooled off, I don't mind wearing layers. 

See how it bows out a bit in the lower back? What causes that, and what modification would I make to prevent that?

I made two mistakes when sewing this shirt, and I want to record them so I don't repeat them when I make the next one. 

1. I used interfacing that was too heavy. I debated between the lightweight and the slightly heavier, and I chose the sturdier one, thinking I wanted the collar to have a bit more structure. That would be fine for the collar, but the whole placket is interfaced and when I sit down, it wants to stay standing up. It isn't a huge problem, but it would be better if the placket were softer and more flexible. 

*Note to self: always match the interfacing weight to the fabric weight.*

2. I forgot to lengthen the placket pattern piece! I added 2 inches to the length of the shirt, but forgot to change the facing. I just sewed on extra fabric to the bottoms of the facing pieces, which no one can see since they're on the inside of the shirt. But there are two extra seams in there.

Even though my expression says, "Stop telling lame jokes," I really like this shirt. I'll show you my second completed basic item next time (and tell you what I failed to make for the third garment).