Cast On Problems

I have completed the Triangular Summer Shawl in the Nancy Bush book “Knitted Lace of Estonia”. The pattern called for the yarn to be doubled and to cast on 251 st. in the knitted on method, then continue with just one strand. My cast on edges are so tight that I almost couldn’t get it blocked. I did not treat the doubled cast on as one stitch - is that my problem? I’m willing to try this pattern again except for my problem with the cast on rows. Should I use a different cast on?

Yes, you work with the double strand as if it were just a single. The knit CO is fine and the doubled yarn is fine, you’re just supposed to end up with 281 sts, not 572. You might want to CO with a larger needle next time, that should make it looser.