To answer your question, you don't know their wt. I did a little experiment with cattlemen that are experienced and feel they can guess wts real close. We weighed the cows and calves put them in a pen walked them around and asked them to estimat- and write it down. They were way off.
We weigh cattle all the time, all registered calves must be weighed for the data books, our feeder calves are weighed monthly so we can adjust their feed, we use wt. to administer the wormer accurately. Just think of the money that can be saved by giving just enought wormer? That alone will pay for the scale over a period of time if you have enough cows. Some local cattlemen's chapters buy a scale and then lend it out to members - good idea.
Billy