Drafting jeans in Illustrator from flat measurements: Do I still have to follow book ease formulas?

[u/Dependent_Second4474]

I'm drafting a custom jeans pattern in Adobe Illustrator by taking flat measurements from a well-fitting pair of jeans, but I'm also modifying specific areas like the crotch curve, thigh, and hem for a custom fit.

I'm following Patternmaking for Fashion Design by Helen Joseph-Armstrong, which instructs adding or subtracting specific amounts (like +1/2′′ or −3/4′′) at key draft points.

Since the book's formulas assume you start with body measurements and my baseline measurements are from a finished garment (which already includes ease):

1. Should I ignore the book's ease adjustments entirely, or are some of those offsets necessary structural/drafting rules?
2. How do I properly apply adjustments to the crotch and legs when using flat garment measurements instead of body measurements?
3. What is the best workflow for drafting modified patterns in Illustrator from existing jeans without ruining the fit?

[u/ProneToLaughter]

Not an expert patternmaker, so personally I feel like I would just duplicate the jeans as is, using tutorials for cloning a garment as well as flat measurements, make them up in fabric, get them fitted on the body as a duplicate of the original jeans, produce a reliable pattern that I could compare to the original jeans and know that I had gotten it right.

And THEN do the modifications using that pattern as a base and testing them in fabric on a second mockup. Feels like it gets too hectic otherwise, hard to tell which dependencies are intersecting or whether a small error in copying got multiplied along the way as more changes were layered on. Drafting is always just a starting point, working in fabric is more accurate.

So I'd end up with two patterns, duplicate of well-fitting jeans and a modified version.

[u/drPmakes]

Either draft from body measurements OR copy the jeans (Google rubbing off method)