In which something odd happens to our seamstress!

So I came home from school and began working on a new buckram hat form. There I was, nestled on the couch with a variety of cats, hand sewing the crown to the side crown when, lo and behold! the eye of my curved needle broke! At first, I thought I pulled too hard and snapped the thread (which in itself is hard to do because I use button thread), but no, the eye of the needle itself broke! LOL! Not having a spare curved needle sitting around, I had to pop to Michael's and get more.

Asking for advice...

I've decided to perhaps do a rusched detail on the underside of the brim so that when I turn it up, you can see it. However, I'm not sure how to calculate the dimensions of the pattern so that when I pull the threads, it "shrinks" down to the right size and shape. Any tips? This has always confused me!