I got a comment from a very helpful reader who suggested that the reason I may have lost so much length was that I used the 3/8" tubing instead of the much less bulky hoop wire or hoop boning. I think she is correct.
I was getting concerned that the hoop skirt would be too short and create a shelf effect at the hem of the over skirt. I tried layers two heavy skirts over the hoop to see if that forced the hoop's hem down significantly:
These skirts were not designed for any hoop or bustle, so the fit was not great, but I was interested in the effect of their weight. I measured and they pulled the hoop hem down about 1". I then draped some of the skirt fabric down the back and decided that I was likely to get "hoop ribs" and that I needed to lengthen the hoop skirt.
Being short as I am, I almost never have to lengthen anything. Let us bask in this moment... ahh...
So the pattern instructions point out that this garment is underwear, which means that you can do some pretty drastic alterations to it after construction. Would it be better to do these things to the pattern? Sure, but hey, who's going to see it?
So, I gritted my teeth and ripped open the front seams to just past the lengthen line. I then cut along the lengthen line and added in a strip of 6" wide fabric (I was shooting for 4.5" - 5" extra length). The instructions do encourage you to remove the hoop wire before sewing in the extra strip. This is sage advice that I did not follow. It made sewing in some bits a little tricky, but I made it. I really can't believe this worked. There's no reason it shouldn't have, it just felt so... wrong.
Then I decided to make a bustle pad to help support this bustle and possibly skirts that do not require a bustle. This took all of 15 minutes.
Here is the hoop first without the pad, then with the pad.
Not a bad evening's work, I'd say. I think I'm done tweaking this for now.
Next up (really): burgundy underskirt.