Personally, I have always just eye balled it. I pull a rat out of one of the racks that looks about right, or thaw one that looks about right. Honestly, most of the time I am just grateful IF they eat! If it comes down to going to a pet store to get a feeder, I would strongly recommend picking it out yourself. If they wont let you then refuse until you get what you want! I am sure the scale thing is a good thing for a perfectionist, but I dont have time for that! This is something I would call an "internet fact". It must be that way because everyone on the internet says so!