Eric,
I had the upholstery on my 01, 420 redone last Fall.
After checking online sources, and local upholstery shops, who wanted several thousand dollars, I got in touch w/ a local lady (recommended by a friend) who does upholstery, etc. in her home.
She came to boat, measured, and we then went to a local upholstery shop
( that she uses frequently) to pick out fabric.
There are hundreds of different fabrics; so your preference and your wallet will dictate your selection.
We bought enough yards for the job, plus 3-4 extra yards to keep for future repair or replacement of those cushion(s) which are subject to most wear.
She did great work, and the total of materials and labor ended up being less than half the lowest prior quote.
So ... my suggestion is ... don't worry about trying to figure your yardage or fabric type, just find a local seamstress, let her measure ... and help you w/ fabric selection. You'll get a great job, support someone who really needs the work, get just what YOU want, and get it for far less than any other way.
And ... she thur-in a few, matching, throw pillows, just to say "thanks for the work"...
