Fly fly away

When I looked at patternreview.com for the pants I'm making (Simplicity 2700), some of the reviews warned me about the bad instructions for the fly. I figured, since I'm making a muslin, I may as well muslin up the fly so I can figure it out. Boy am I glad I did! Some issues were just unclear instructions, but there are a couple subtleties/changes I want to make. Picture heavy post, so everything else is after the break...


For reference, I started with a pair of ready-to-wear slacks*:

* Please forgive the wacky exposure etc on all these pictures, I took bad photos and had to try to get the necessary detail in post-procesing.

Without further ado, here's my edited instructions:

1. Sew on the fly facing to the right front, with appropriate pressing. I'm thinking I may try to make a grown-on fly instead, to avoid worrying about the seam showing. Although the seam may be adding some stability... Maybe I should just stabilize the zipper with silk organza anyways.
2. A couple steps I skipped on the muslin: interfacing the fly facing, finishing the edges of the fly facing, understitching.
3. Here's where my confusion started. The directions say to put the zipper face down and line up the zipper 1/4" past the seam. Below, I've shown how I interpreted that:
The pins are along the intended stitching line. Now, I don't really trust lining it up this way. Do all zipper tapes have the same width? It did turn out well, so maybe it's right, but...
Here's what it looks like with the zipper open:

(Ignore the "back" label, I reused muslin fabric.) The pink line is that 1/4" from the center front that I used to line up the tape of the zipper. It looks to me like I can just stitch the zipper teeth to be 1/4" from the center front on the fly facing, which I think sounds a lot easier
4. One note on stitching the zipper to the fly: I think it works better if you push the top of the zipper to the right (away from center). When the zipper is closed, the slider takes up some room up there. If the stitching is straight, this creates an unsightly gap at the top, where it looks like you're too big for your pants. If the zipper is pushed away from center here, it makes room for the slider and the pants sit closed better. One way to accomplish this is to make sure to have the zipper closed when stitching, and keep the left side straight. This will create the distance the slider needs because it will be there.
5. At this point, the directions say to stitch the topstitching line that basically keeps the fly facing in place (the curved line). I did this, and then had to unpick it when I needed to fix the zipper. What's the point of basting a zipper before stitching it if you don't test closing it before it's committed? I think that step can be done last. If you really need to temporarily keep it closed, baste or safety pin it.
6. On to the left side!
Press under 3/8" (I may have this measurement wrong), leaving 1/4" past the center front. The directions (I think, it wasn't totally clear) say to align the edge of the zipper with the fold, but I worry a little about fabric getting caught in the teeth, so I aligned the fold with the edge of the teeth, and then stitched as close as I could. I think I'd prefer to leave a little more of the fold to the left of the stitching for a more secure hold. Maybe 1/8", so it splits the fold-center front distance down the middle. Oh, and I think this is technically a "baste" step, not stitching yet.
7. This is where I might do the topstitching. Well, first try them on, see how the zip looks closed, etc. Now that both sides of the zipper are attached, you can see what changes are necessary. Adjust the basted zips as seems appropriate, then topstitch the fly facing to the right front. Or, leave it to the end and make sewing the crotch seam easier.
8. Finally, the underlap. The instructions say to fold it in half and stitch around the edges. What this ends up doing is leaving a bunch of unfinished edges. I don't mind the top edge being unfinished, since it'll get caught up in the waistband, but why not make a clean fold and have finished edges! Here it is attached:
It's almost no extra work to contain those edges inside that fold, so why not do it? Also, for some reason the pattern says to cut 2 of the underlap, but I couldn't find a place where you're supposed to use the second. I really feel like I'm missing something obvious, but at any rate, here's my hack:
9. Extend the bottom and sides of the underlap by 5/8". Stitch it, right sides together, like a bag, stitching the bottom and side edges. Turn inside out, clipping seams as necessary. This will result in a piece the same size as the underlap the pattern calls for, but with the bottom and side raw edges nicely contained.
10. Now just follow their instructions: Align the seam line (5/8" inside the edge of the underlap) with the stitching/basting on the left zip and sew (baste first if necessary).
11. Last step! Tack the underlap to the fly facing by hand, so it'll stay closed inside your pants.
12. OK, really the last step. If you didn't topstitch the fly facing down before, this is where it needs to be done. I think, though, that sewing the crotch seam up to the circles first may be easier. I had to stitch up to the circles by hand because the fly facing was in the way...

And voila! I do still want to go through the muslin and make sure all my exposed edges will be finished a la the underlap.