Ashley here. I wanted to explain how I weigh my roving (technically this is “combed top” for all you fiber heads out there) before the dye process.
I like working with 4 ounce portions if fiber. When I first started out, you would have LOL’d or even started ROFL upon seeing my long rows of fiber stretched out along the kitchen floor, the couch, or the bed. I would try to get all the lengths exactly the same so that I could guess-timate the ounces before ripping each edge of roving apart to make my ~4 oz pieces.
I’ve gotten smart since then. I bought a postal scale! I found one on eBay for $20-$30 and decided it was worth it, especially since I am running my own Etsy shop, Dye2Spin
I quickly learned that I can’t just pile the dang fiber on top of the little platform of the scale, it all falls off! So you’ll need a clear container of some sort to put your fiber inside of. I use a glass bowl but plastic will work as well, as long as you can see through it or as long as it doesn’t cover up the tiny screen on your scale, it will work just fine.

It’s pretty simple really. The scale must be turned on AFTER the bowl is placed on top. This ensures that the scale is at zero instead of showing the weight of your container. You might also have a “tare” feature on your scale which will do the zeroing out for you. You place your fiber in the container sitting atop the scale like so.

You begin unraveling or piling or putting or hoping your fiber into the bowl. By the way, if any of you can stare at your mass of fiber and HOPE it into your container, I want to meet you and get your autograph.
When you’ve reached your
desired amount of fiber, you break it off. Breaking the fiber off includes you pulling with both hands on either side of the point of separation. Never ever use scissors to cut your fiber into pieces unless you have a specific or desired effect in mind that only cutting can provide.
If you intend to spin this fiber, give or sell for spinning, a cut end of roving will only create enemies, not BFFs (Best Friends in Fiber).
This gives you the amount of fiber you’d like to work with sans the long strips of roving laying on the floor, collecting every little dog or bunny hair there happens to be, just waiting to be swept up with a Merino wool “broom.”

There’s my pit bull, Cinderbelle, and Jersey Wooly rabbit, Phil (short for Xenophilius), the first day they met.
I hope this small tutorial helped you see the light in one method of weighing fiber. Now I’m off to dye like a son-of-a-gun!!