Plan:
1. Copy existing jacket pattern to tissue paper. Will use tracing paper...
2. Build a jacket using some scrap stretchy fabric of some sort (not the fancy stuff, not exact stretch, but hopefully close), minus zipper!
3. Check fit.
4. Adjust fit, deconstruct adjusted jacket.
5. Transfer adjusted jacket to pattern
6. Build jacket with new fancy softshell.
Getting the collar length is going to be a bit of a challenge.
And I think the bottom of the jacket probably should be a straight line... I think I flipped the wedge part. I will have to watch that.... I guess I can cut the jacket slightly long and adjust the bottom, however, as it needs to fold back over on itself.
Someone's video on shortening a separating zipper.

@ai6yr open-lower-back wedge cut, tied with strings.
Go kink/medieval on that bitch.
@ai6yr DOOOOOO EEEEEEEEEEET. You WANT IT.
Search your feelings. You know it to be true.
@ai6yr ::stares::
WELL..... ???
@ai6yr BTW one of the weirdest but comfiest pieces of clothing I own is an odd combo/mix of rain shell and a short cloak. It has wide sleeves, a hood, and closes by buttons front of shoulder/collar bone, one of those turned inside (told you, weird)
It looks like something out of a new Star Wars movie set. I'd wear it more often, but I don't want to look TOO MUCH like a scifi movie extra, walking around.
@ai6yr make the collar a reversible hood.
The fastening in front must be loop n peg. Bonus for the teeth or claws of whatever tried to kill you last.
Hoodie ... I made a hood for a knitted sweater and ended up with pointy hood, and regretted it. Fortunately it is red, not white, but still I wear it with the pointy part tucked in.
Next time, I would make it as a rectangular strip up the back of the neck and over top of head, and two other pieces, one on each side of head. More seaming, but fits and looks a lot better.
@EugestShirley My midsection is growing! 🤪
In 1996 I believe I weighed in the range of 165... I was extremely tall and skinny. I now am 195 to 200, lol.
This is one case where the bottom hem might not be straight across.
Esp for biking jackets, the back might be longer.
However, the jacket you seem to be using as model seems to be an even-hemmed (horizontal, not diagonal) jacket. Look at biking rainshells, and other jackets, and pick a design.
Maybe put the longer back on a rainshell, and leave this jacket horizontal? It's up to you, but this is what I would probably choose.
I'm still working off the original roll I got with a sail kit years ago
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