How to estimate yarn for edging

I just bought Nicky Epstein’s Knitting on the Edge. I am on a shawl kick and wanted to try edgings that would challenge my skills - but I am stuck on figuring out how much yarn to buy for the edging. I had planned to just make a shawl in a size and shape I know, varying the main body with various stitches and wanted to just add edging - but I am afraid I will get 3/4s of the way around and run out of yarn. I suppose I could do geometry and figure the area of the edging vs the area of the shawl and then do algebra to figure out the missing number but isn’t there an easier way???

I always over buy for projects on purpose, that way if I make a mistake I have plenty to start again or fix the porblem. For instance, I made a quilt over the summer, and instead of buying the called for yard and a half of a certain fabric, I bought 3 yards. When a sweater pattern calls for 5 skeins of yarn I buy 8… If I have tons left over I can return what I didn’t use to the store, or just add it to my stash for a new project later. This is especially important because while I could always go back and get more yarn, even if I find the same dye lot it might not match perfectly. I guess this doesn’t really answer your question, but I try to only do projects that do not require math. :roll:

I’m like Hildegard, I always buy an extra ball or skein for my projects. There is always something you can do with the leftovers!

I’d say try and figure out how much yarn you’ll need, then buy 2 or 3 extra balls.

Good luck!