@tnp -- Sadly, it doesn't work quite like that. Mostly because -- with the haircut I was aiming for, most of what determines the length is the back of my hair. And with my head tilted down, it's really hard for me to see in the mirror. With my head tilted down and her cutting the back of my hair, it's impossible, mostly because of that whole not having eyes in the back of my head.
And in this case, in an attempt to get the back of my hair to lay the way I wanted it to? She was apparently having problems getting it to do that. She didn't tell me that, though. She just kept cutting. And cutting. And cutting until it was short enough to lay that way. By the time she was coming around to the front and I could see where the scissors were and going "Uhhhh, Katie, why is there a giant lock of cut hair on my shoulder? What are you doing?" my options at that point were let her finish the cut or stop and have what would've basically been a reverse mullet -- party in the front, business in the back.
So not me, but basically my hair now:

Pinterest board I brought with me, to commiserate with @Cupcake and @Auspice having shitty experiences.
On the upside, I've realized over the last two days that is an objectively cute haircut and does look nice on me. In fact, it's exactly what I wanted for shape and style.... just much, much, much shorter. Like, I can't even pull it back short. And as @mietze said, I was trying to grow it out, so losing that much hair all at once means it's going to take about a year to grow back. If this -- and the other fuck up that happened while my regular stylist was on maternity leave -- had not happened? My hair would not be chin length right now. It would be several inches past my shoulders, which is what I was hoping to have eventually, and had just gotten it to the 'slightly past my shoulders' point.
Super frustrating. 