So I did the FBA as directed in Tasia's Lonsdale sewalong, and it worked much, much better than my original attempt. But I still had a few things that needed work. Exhibit A: The back was doing this weird diamond-shaped wrinkle thing. I didn't take any pictures of the muslin once I'd fixed it, but here's what I ended up having to do:
There ended up being a difference of about an inch between the original pattern width and what I ended up with after the FBA, but the dart that I needed to add in front ate most of that up. I still needed to add about 3/8" to either side of the waistband after pinning and fiddling with the pattern pieces to "walk" the length of the waistband. So then I added half of that to all of the side seams on the skirt, as best as I could estimate between 1/8" and 1/4" on my gridded ruler. I'm not too worried about fitting the skirt, since that's going to be much easier to adjust as I go.