When my spindle was full and I had to wind off one cop of singles the other day, I weighed my bag of wool top (I didn't weigh it before I started, but I trust the shopkeeper to sell me very close to what I asked for -- and if there is a variation, it will be to the good). 3 oz., so I figured I had spun about an ounce. For the second cop of singles, I carefully weighed my top and made sure it was an ounce, since I wanted the cop sizes to match.
My spinning is regular enough that there was only about 9" difference in the two cops' lengths.
Oh. My. I am so proud.