Any help recommending appropriate upholstery for my early George Nelson sectional.  

Girard + Nelson
I recommend the HM reissue of Alexander Girard's Lanalux (100% wool). This textile is so soft you may mistake it for sitting on a live sheep. This material is in a very high price category but if you want to keep your re-upholstery job authentic this is the way to go.
If you order the material through a Herman Miller dealer (not a retailer like DWR) you should get the best price.
https://www.hermanmiller.com/CMF/CDA/DetailView.jsp?color_id=2704

Thanks!
Thanks so much for your suggestion. I will definitely check it out.

Determining how many yards....
Any suggestions for determining how many yards of material I will need?
Thanks!

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You start measuring the pieces of fabric on the couch. That has enough pieces you may need to measure and draw them on paper. Then you need to know a few things about the fabric: how wide is it? does it have a grain you have to follow? Are the front and back different? And you figure out how to fit the pieces on the fabric as tightly as possible.
Are you going to be sewing the upholstery?

thanks
Fortunately, it's a solid pattern so I won't have to worry too much about the width of the fabric. I haven't decided if I'm going to tackle this job myself or not. Ultimately, I think I should defer to an upholsterer but I'd rather bring the material to her rather than have her order it etc.
Thanks for the input!

Depends on the Upholsterer
Some upholsterers charge an extra fee if you supply your own material (usually by the yard). I would get quotes before committing to any one textile. If you plan on doing it yourself keep in mind some textiles are much more difficult to work with; loose weaves commonly found in mid-century textiles tend to fray easily and require a delicate hand (I have a Tuxedo bench done in Linomix and I'm still picking threads off of it), heavy weight textiles may be more difficult to shape and fit to the piece.

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The reason I asked whether you are doing it yourself is that I would consider it respectful towards your upholsterer to ask their opinion and follow it with regards to the yardage. (You could of course calculate it yourself and use your calculation to determine which upholsterer to go to, and that might be a good way to decide as long as your calculation is correct)
If I were a professional upholsterer and you brought me a sofa to restore, with what I regard as too little fabric, I would be apt to turn the job down, or charge you more than usual for the annoyance, order the right amount of cloth myself and stick your yardage in the back room. (Don't want dye batch color mismatch in the middle of the sofa because a few yards came from a different bolt).
And when I do upholstery for myself, I very much do concern myself with the width of solid cloth, because seams down the middle of a cushion are not acceptable.

Good to know
Thanks!

Uphosterer
I think I'll be speaking with my upholsterer. She is usually flexible about bringing your own fabrics.
Thanks!