HI! I'm new to this page, so not sure how/where to start a new thread, or if I should, so please correct me

I'm taking on a blanket project and writing my own pattern. I've got that part. The fun part is figuring out how much yarn i'll need to buy. It's a paid project, so I want to provide as accurate as a price to my customer as I can to keep from over or undershooting by too much.

I think I may have figured it out, but being it's my first time, it'd be awesome to get second opinions. Here's what I know based on a test swatch:

It'll be 6 foot wide x 4 foot tall (the 6 foot part is a must, the 4 is flexible up to 4.25 feet). It's knit with 2 strands at the same time.

a 4 x 4 inch swatch yields 14 stitches wide x 21 stitches tall.

if i knit a row of 14 stitches and mark the starting point and ending point on the yarn, and unravel that, I get 15 inches of yarn used. If I wrap the yarn 14 times on the same needle, I get 12 inches. (?) So I went with 15.

I calculated 252 stitches wide x 256 (or 252.. 256 comes out better) tall based on those above numbers/inches/stitches.

With all that said, I got 270 inches of yarn per row X's 256 rows = 69,120 inches. convert that to feet, you get 5,760. convert that to yards, you get 1920. There's 355 yards per skein, so divide that out and I get 5.4 or 5.5 skeins. Since i'm knitting with 2 strands at a time to thicken it, i think you multiple 5.5 x 2 and get 11 total skeins.

am I close?!

Thanks!