Mine too! I feel like it's the best way to avoid wasting yarn. As for your question, if I'm starting with a skein/anything hard to pull from, I make it into one ball. Then I borrow my husband's kitchen scale, tare it with a yarn bowl, and weigh the ball of yarn in grams. Leaving the ball on the scale, I hand roll a ball from the end until the number on the scale is half of the original number. I even swap the balls to make sure they weigh the same. Then I cut the yarn between the balls and just hope my gauge stays consistent enough to use up the yarn at the same rate!

I also do it with a kitchen scale. I weigh the whole ball and then wind two smaller ones based on that weight so I have two (almost) identical ones in weight. I also try to keep an eye on where the colors change so I start both balls with the same colour change if it is self striping.