Estimating Yardage Isn't My Forte

I've been working on the intarsia version of the Swingin' baby blanket, and I'm now thinking I'll need to order more yarn.  This is frustrating on many levels:

1. I already ordered more yarn once - 3 skeins (540 yds) more! - and I had thought I was being generous in that estimate.
2. Before I sent in the first re-order, I'd already decided to modify my sample from the original pattern, eliminating a whole 40 rows from the blanket!
3. The shipping is expensive, and since I'm not super-thrilled with the yarn, I don't want to order more than is necessary, even though that would reduce my per-skein cost for postage (something I actually do think about).
4. I'm nervous that this is just foreshadowing for my next project, a baby blanket for which I received yarn support based on my yardage estimates ... and what if I screwed those up this badly?  I can't very well go back to the yarn company and say, "oops!" can I?
5. And then there's the whole embarrassment factor.  I have a math degree and was a high school math teacher.  I have pretty good estimation skills.  But I seem to have a mental block for figuring these things out, and I really just don't understand why.

Thankfully, I have some test knitters who have been working on this project in a variety of yarn weights and so are able to provide their actual yardage counts.  I'm so grateful that I'll be able to use their work to provide accurate material requirements in the finished pattern.  I can't imagine how stressful it would be to publish a design without having had yardage calculated from a finished object ... not after this experience!

(Plus, I'm heartened by the fact that I'm not the only one who has this problem ... this blanket is going to my cousin for her soon-to-be-born baby.  She's also knitting an afghan, and she's also had to place three orders for yarn!  Maybe it's just going to be the way with this babe!)